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I00M/5-79 


PLATE  I. 
Optics  of  the  Compound  Microscope.1 


F, 
A 
O, 
03 
04 
EP 


Upper  focal  plane  of  objective.     F2   Lower  focal  plane  of  eyepiece. 
Optical  tube  length  =  distance  between  Fi  and  V2. 
Object.     02   Real  image  in  Fj  transposed  by  collective  lens. 
Real  image  in  eyepiece  diaphragm. 

Virtual  image  formed  at  the  projection  distance  C,  250  mm.  from  EP. 
Eye-point.      CD    Condenser  diaphragm.      L   Mechanical  tube  length 
(160  mm.). 

1,  2,  3   Three  pencils  of  parallel  light  coming  from  different  points  of  a  distant 
illuminant. 


1  From  "The  Microscopy  of  Drinking  Water"  by  George  C.  Whipple, 
courtesy  of  the  author  and  that  of  the  Bausch  &  Lorab  Optical  Co. 


Reproduced  through  the 


ELEMENTARY 
CHEMICAL  MICROSCOPY 


BY 

EMILE  MONNIN   CHAMOT,  B.  S.,  Ph.D. 

Professor  of  Sanitary  Chemistry  and  Toxicology,  Cornell  University 


FIRST  EDIT/ON 

FIRST    THOUSAND 


NEW  YORK 

JOHN    WILEY    &    SONS,   Inc. 

London:    CHAPMAN   &  HALL,   Limited 

1915 


Copyright,  1915, 

BY 

EMILE   MONNIN   CHAMOT 


Stanbopc  iPress 

F.    H.GILSON    COMPANY 
BOSTON,  U.S.A. 


PREFACE. 


The  American  chemist,  usually  ready  to  accept  with  alac- 
rity all  time,  labor  and  money  saving  devices,  has  been  strangely 
backward  in  taking  advantage  of  the  benefits  to  be  gained 
through  the  intelligent  application  of  chemical  microscopic  meth- 
ods in  the  industries  and  in  research.  He  has  also  failed  to 
grasp  the  fact  that  the  modern  microscope  is,  in  reality,  a  more 
important  adjunct  to  his  laboratory  than  spectrometer,  polarim- 
eter  or  refractometer;  in  fact,  it  may  be  said  that  the  micro- 
scope is  entitled  to  as  important  a  place  as  the  analytical  balance. 
No  one  other  instrument  can  perform  so  many  functions  and  do 
them  all  well. 

This  curious  reluctance  to  grasp  the  opportunities  offered  is 
the  more  extraordinary,  when  we  recall  that  the  earliest  com- 
prehensive work  dealing  with  microchemical  methods  was  from 
the  pen  of  an  American  —  Theodore  G.  Wormley  --  whose 
classic  "The  Microchemistry  of  Poisons"  appeared  in  1867. 

The  failure  of  the  chemists  to  obtain  from  the  microscope  all 
that  the  instrument  is  capable  of  yielding  is,  perhaps,  largely 
due,  first,  to  the  fact  that  few  of  them  are  given  an  opportunity 
of  becoming  sufficiently  familiar  with  the  instrument  and  its 
accessories;  second,  they  are  not  aware  of  the  great  variety 
of  problems  which  are  solvable  through  the  microscope,  nor  of 
the  specific  sort  of  problems  for  the  investigation  of  which  this 
is  the  instrument  par  excellence;  third,  there  has  been  a  lack  of 
elementary  manuals  covering  the  field,  and  for  this  reason  the 
microscope  has  been  looked  upon  as  an  instrument  peculiar  to 
the  biological  laboratory. 

One  application,  if  no  other,  should  appeal  to  every  chemist, 
that  of  microscopic  qualitative  analysis,  because  of  its  enormous 
saving  of  time,  labor,  material  and  space,  yet  with  increased 
sensitiveness  of  tests  and  greater  certainty  of  results. 

iii 


IV  PREFACE 

The  very  apparent  need  of  including  a  course  in  the  manip- 
ulation and  applications  of  the  microscope  in  the  curriculum  of 
students  of  chemistry  led  to  the  establishment,  by  the  author, 
of  laboratory  courses  in  chemical  microscopy  some  fifteen  years 
ago.  These  courses  have  comprised  informal  lectures,  demonstra- 
tions and  laboratory  practices.  The  students  have  been  guided 
by  their  notes  and  by  mimeographed  and  typewritten  sheets. 
With  the  growth  of  the  courses  in  number  of  students,  apparatus 
and  laboratory  equipment,  some  more  permanent  and  compre- 
hensive outline  has  become  imperative.  The  result  has  been 
the  preparation  of  the  present  little  book.  The  author  has 
intended  it  primarily  for  his  students  in  elementary  chemical 
microscopy  and  as  a  basis  for  more  advanced  work  in  specific 
fields,  but  he  hopes  that  the  gathering  together  of  methods 
and  apparatus  may  prove  of  value  to  American  chemists  at 
large  and  perhaps  serve  to  arouse  in  some  an  interest  in  one  of 
the  most  fascinating  branches  of  chemical  science. 

The  actual  nucleus  about  which  the  various  parts  of  the  book 
have  grown  is  a  series  of  some  twenty  articles  written  by  the 
author  between  the  years  1899  and  1902  for  the  Journal  of  Applied 
Microscopy,  dealing  with  methods  of  microchemical  analysis;  to 
this  foundation  have  been  added  the  laboratory  direction  sheets 
and  the  substance  of  the  lectures  delivered. 

Until  the  year  1 911,  when  Emich's  excellent  little  Lehrbuch 
dcr  M ikrochemic  appeared,  there  was  not  in  existence  any  work 
embodying  the  broad  applications  of  the  microscope  to  the 
solving  of  problems  such  as  arise  in  the  chemical  laboratory. 
So  far  as  the  writer  is  aware  this  is  the  only  book  touching  this 
field.  The  topics  presented  by  Emich  are  substantially  those 
which  have  been  covered  in  the  author's  courses  with  the  excep- 
tion that  more  weight  is  placed  upon  analytical  methods  and 
less  upon  apparatus.  The  present  writer  therefore  feels  that 
there  is  still  room  for  an  outline  of  Chemical  Microscopy  proper. 

It  is  assumed  that  the  students  for  whom  this  textbook  is 
intended  have  had  a  course  in  crystallography  and  one  imphysics, 
including  optics.  Therefore,  only  a  mere  statement  of  funda- 
mental facts  has  been  thought  essential,  that  is,  only  so  much 


PREFACE  V 

as  is  necessary  to  recall  knowledge  already  acquired  but  not 
yet  applied  in  practice. 

In  discussing  the  polarizing  microscope,  only  the  barest  pos- 
sible outline  of  its  use  and  application  has  been  thought  wise. 
This  chapter  is  intended  to  be  largely  suggestive  in  character 
and  to  induce  at  least  some  students  to  extend  their  studies  to 
include  optical  crystallography  and  petrography. 

In  the  chapter  dealing  with  grinding,  polishing  and  etching, 
it  was  found  impossible  to  properly  present  the  subject  without 
unduly  enlarging  the  book  and  encroaching  too  deeply  into  the 
field  of  microscopic  metallurgy;  only  the  most  fundamental 
methods  of  alloy  treatment  have  therefore  been  given. 

The  instruments  figured  (and  the  methods  described)  have  all 
been  tested  and  tried  by  the  author  with  but  one  or  two  excep- 
tions. The  instruments  are  those  with  which  the  Cornell  Uni- 
versity Laboratories  are  supplied  or  those  which  have  kindly 
been  loaned  by  their  makers.  Doubtless  there  are  other  pieces 
of  apparatus  and  other  instruments  which  may  be  as  satisfactory, 
but  it  has  been  thought  best  to  discuss  only  such  as  have  actually 
been  examined  and  tested  experimentally  by  the  author  and  his 
students. 

For  the  benefit  of  those  who  may  wish  to  obtain  similar  in- 
struments the  manufacturers  have  in  most  cases  been  indicated. 

In  preparing  such  an  outline  the  work  of  an  author  must  of 
necessity  be  largely  one  of  compilation,  of  modification  of  old 
methods  and  the  presentation  of  old  ideas  from  a  new  viewpoint. 
The  present  writer,  therefore,  makes  no  claims  for  originality,  and 
as  a  student  of  that  remarkable  teacher,  the  late  Professor  Behrens 
of  the  Polytechnic  School  of  Delft,  he  naturally  has  followed  and 
favored  the  methods  developed  by  this  master  of  the  art  of  the 
qualitative  analysis  of  minute  quantities  of  material  and  he 
acknowledges  fully  his  indebtedness  to  his  former  teacher,  and 
takes  this  opportunity  of  expressing  his  gratitude  for  the  advice 
and  help  given  him  by  his  guide  and  friend. 

To  Simon  Henry  Gage,  Professor  Emeritus  of  Histology  and 
Embryology,  the  writer  also  acknowledges  his  indebtedness  for 
much  that  is  here  presented.     It  is  largely  due  to  the  spirit  of 


VI  PREFACE 

optimism  and  love  for  research  with  which  this  indefatigable  in- 
vestigator is  ever  surrounded  that  the  author  was  originally  led 
to  enter  the  held  of  applied  microscopy  when  first  a  student. 

To  Professor  Louis  Munroe  Dennis,  Head  of  the  Department 
of  Chemistry  of  Cornell  University,  the  writer  is  even  more  in- 
debted in  later  years  for  his  unflagging  enthusiasm  and  confidence 
in  the  possibilities  of  a  neglected  field.  Without  his  encourage- 
ment and  support,  the  development  of  laboratories  and  equipment 
would  have  been  impossible  and  the  preparation  of  this  little  book 
impracticable. 

The  author  also  wishes  to  express  his  indebtedness  to  Dr. 
E.  Mace  of  the  University  of  Nancy,  France,  and  to  colleagues 
in  the  Cornell  University  departments  of  chemistry,  physics, 
and  mineralogy  for  valuable  advice  and  suggestions.  His  thanks 
are  also  due  to  his  assistants  Dr.  C.  M.  Sherwood  and  Mr.  H.  I. 
Cole  for  reading  manuscript  and  testing  methods. 

E.  M.  C. 

Ithaca,  N.  Y.,  June,  1914. 


CONTENTS. 


Optics  of  the  Compound  Microscope Frontispiece 

CHAPTER  I. 

Objectives  and  Oculars. 

Page 

Function  of  the  objective i 

Designation  of  objectives i 

Working  distance  of  objectives 2 

Different  kinds  of  objectives 2 

The  draw-tube 4 

Angular  aperture  of  objectives 4 

Numerical  aperture  of  objectives 5 

Immersion  objectives , 5 

Variable  objectives 6 

Resolving  power 6 

Illuminating  power 8 

Penetrating  power 8 

Selecting  objectives 8 

Care  of  objectives 9 

Function  of  oculars 11 

Negative  and  positive  oculars n 

Eye-point 12 

Different  types  of  oculars 13 

Care  of  oculars 14 

Limit  of  magnification 15 

CHAPTER  II. 

Microscopes  for  Use  in  Chemical  Laboratories. 

Specifications  for  chemical  microscopes 17 

Microscopes  for  general  chemical  microscopy 19 

Large  stage  microscopes 22 

Comparison  oculars 23 

Comparison  microscopes 25 

Hot  stage  microscopes 28 

vii 


Vlll  CONTENTS 

CHAPTER   III. 

Illumination  of  Objects;  Illuminating  Devices. 

Page 

I  'liferent  modes  of  illumination 30 

I  ransmitted  light 30 

Condensers,  Abbe  condensers 32 

Reflected  light 37 

Dark-ground  illumination 40 

I  'ark-ground  illuminators 41 

Objectives  for  use  with  dark-ground  illuminators 43 

Resolving  power  with  dark-ground  illuminators 44 

Adjustment  of  dark-ground  illuminators 46 

Orthogonal  illumination 50 

Differential  color  illumination 51 

Ultraviolet  ray  illumination 51 

Fluorescence  microscope 52 

CHAPTER   IV. 

Ultramicroscopes;  Apparatus  for  the  Study  of  Ultramicroscopic 

Particles. 

The  principle  of  the  ultramicroscope 54 

Brownian  motion 55 

The  diffraction  images  of  ultramicroscopic  particles 55 

The  slit  ultramicroscope  and  its  adjustment 57 

Reflecting  condenser  ultramicroscopes 65 

The  cardioid  ultramicroscope 67 

The  reflecting  prism  ultramicroscope  of  Cotton  and  Mouton 69 

The  Jcntzsch  reflecting  condenser 71 

The  immersion  ultramicroscope 72 


CHAPTER    V. 

The    Examination    of   Opaque  Objects;     Vertical   Illuminators; 
Metallurgical  Microscopes. 

Simple  vertical  illuminators 76 

Adjustment  of  vertical  illuminators 78 

Interpretation  of  appearances 80 

Special  forms  of  vertical  illuminators 81 

Maintaining  the  alignment  of  illuminator  and  radiant 87 

Mounting  polished  specimens  for  study 89 

Metallurgical  microscopes,  metallographs 90 

Shop  or  Works  microscopes 99 


CONTENTS  ix 

CHAPTER  VI.' 

Useful  Microscope  Accessories;    Laboratory  Equipment;    Work  Tables; 

Radiants. 

Page 

Drawing  cameras 102 

Drawing  eyepieces 105 

Microspectroscopes 106 

Calibration  of  microspectroscopes no 

Mechanical  stages 113 

Rotating  and  orientating  devices 115 

Lens  holders 118 

Reagent  containers 119 

Rods,  platinum  wires,  pipettes 121 

Spatulas 122 

Forceps 122 

Object  slides 1 23 

Watch  glasses  and  evaporators 126 

Burners  for  microchemistry  investigations 126 

Tongs 1 29 

Work  tables 129 

Radiants 132 

Nosepieces  and  objective  changers 137 

Sedimentation  glasses 139 

The  microscope  as  a  polarimeter 139 


CHAPTER   VII. 

Micrometry;  Micrometric  Microscopes. 

Different  methods  of  measuring  microscopical  objects 142 

Methods  of  direct  comparison 143 

Micrometric  microscopes 144 

Micrometry  with  a  camera  lucida 147 

Determining  the  magnifying  power  of  a  microscope 148 

Micrometry  by  means  of  micrometer  oculars 149 

Determination  of  the  ocular  micrometer  ratio 150 

Step  micrometers *53 

Contrast  micrometers , 154 

Filar  micrometers 154 

Micrometry  by  means  of  a  scale  projected  by  the  Abbe  condenser 155 

Use  of  the  micrometer  fine  adjustment 157 

Measurements  of  thickness 158 


X  CONTEXTS 


CHAPTER  Vm. 

Polarized  Light;     the   Simple   Polarizing   Microscope;     Crystals   under 

the  Microscope. 

Page 

Polarized  light,  its  properties  and  applications 159 

Isotropic  and  anisotropic  substances 159 

The  Nicol  prism 160 

Polarizers  and  analyzers 162 

Testing  the  adjustments  of  the  polarizing  microscope 163 

Centering  the  rotating  stage 164 

Polarized  light  without  a  Nicol  prism 166 

Fundamental  principles  of  crystallography 167 

Elements  of  optical  crystallography 170 

Directions  or  axes  of  vibration  of  crystals 172 

Use  of  converging  polarized  light 1 73 

Polarization  colors 174 

The  selenite  plate 175 

Absorption  of  light,  plcochroism 1 76 

Measurement  of  angles  and  of  extinction  angles 177 

Characteristics  of  the  six  crystal  systems 180 

Experiments  with  crystals 182 

CHAPTER   IX. 

The  Determination  of  Refractive  Index  by  Means  of  the  Microscope. 

The  relation  between  refractive  index  and  contour  bands 184 

Principle  of  the  immersion  method 185 

Behavior  of  air  bubbles  and  oil  globules 186 

Half-shadow  method  of  illumination 188 

Calculating  refractive  indices  of  mixtures  of  liquids 191 

Refractive  index  of  anisotropic  crystals 193 

Uniaxial  and  biaxial  crystals 193 

Determination  of  the  refractive  index  of  liquids 196 

Determination  of  thickness  by  refractive  index 200 

Liquids  for  use  in  the  immersion  method 201 

Crystals  for  use  in  the  immersion  method 203 

Refractive  indices  of  typical  crystals 204 

CHAPTER   X. 

Quantitative  Analysis  by  Means  of  the  Microscope. 

Methods  available 2°5 

Analysis  of  powdered  material 2°6 

Net  ruled  oculars  and  their  uses 2°8 


CONTENTS  xi 

Page 

Counting  cells 210 

Determination  of  weight  by  micrometry 212 

Volume  and  weight  per  cents;  area  measurements 216 

Estimation  of  molecular  weight 216 


CHAPTER  XI. 

The  Determination  of  Melting  and  Subliming  Points. 

Approximate  methods 220 

Exact  methods 222 

Hot  stages 224 

Subliming  points 227 

CHAPTER  XII. 

Methods  for  Handling  Small  Amounts  of  Material. 

Testing  for  solubility 228 

Decantation 230 

The  centrifuge 233 

Filtration 238 

Sublimation 240 

Distillation 244 

Ignition,  fusion 248 

Grinding  and  mixing 249 

CHAPTER   XIII. 

The  Methods  of  Microchemical  Qualitative  Analysis. 

The  various  ways  in  which  reagents  are  applied 250 

Preparation  of  special  reagents 271 

CHAPTER  XIV. 

Characteristic  Microchemical  Reactions  of  the  Common  Elements  and 

Acids  when  in  Simple  Mixtures. 
Cations  : 

Sodium 273 

Potassium 281 

Ammonium 285 

Calcium 287 

Strontium • 292 

Barium 295 


Ml  CONTENTS 

Page 

Calcium,  strontium  and  barium,  additional  tests 300 

Magnesium 304 

Zinc 307 

( ladmium 316 

Mercury 318 

Lead 323 

Silver 330 

Copper 339 

Aluminum 341 

Tin 346 

Arsenic 348 

Antimony 351 

Bismuth 354 

Chromium 356 

Manganese 359 

Iron 362 

Nickel 363 

Cobalt 365 

Testing  for  cations  in  simple  salts 367 

Anions: 

Testing  for  anions  in  simple  salts 368 

Group  reactions  of  the  anions 369 

Acetates 374 

Arsenates 374 

Arsenites 374 

Borates 375 

Bromides 375 

Carbonates 375 

(  hlorides 376 

Chlorates 376 

Chromates,  bichromates 376 

( 'yanides 376 

Cyanates 377 

Ferricyanides 377 

Fern  ><  yanides 378 

Iodates 378 

Iodides 378 

Nitrates 378 

Nitrites 379 

Oxalates 379 

Phosphates 380 

Silicates 380 

Sulphates 380 

Sulphites,  thiosulphates 381 

Sulphides 3&1 

Sulphocyanatcs 381 

Tartrates 381 


CONTENTS  Xlll 


CHAPTER  XV. 

Preparing  Opaque  Objects  for  the  Microscopic  Study  of  Internal 

Structure. 

Page 

Fundamental  principles 383 

Grinding;  abrasive  wheels 384 

Grade  and  grain  of  abrasive  wheels 385 

Selecting  wheels 386 

Speed  of  rotating  wheels 387 

Abrasive  papers 388 

Preparing  specimens 389 

Etching 391 

Etching  liquids 392 


APPENDIX. 

Table  of  melting  points 395 

Periodic  classification  of  the  elements 396 

Books  for  reference 397 

Index 399 


ELEMENTARY 
CHEMICAL  MICROSCOPY. 


CHAPTER  I. 
OBJECTIVES  AND   OCULARS. 

The  modern  compound  microscope,  in  any  one  of  its  many 
complicated  forms  employed  by  chemists,  consists  essentially  of 
three  parts,  (i)  an  objective,  (2)  an  eyepiece  or  ocular  and  (3)  a 
device  for  properly  illuminating  the  object.  The  manner  in  which 
these  three  essential  components  are  mechanically  mounted, 
and  their  relative  importance  with  respect  to  each  other  will  de- 
pend upon  the  nature  of  the  investigation  to  which  the  instru- 
ment is  to  be  specifically  applied.  The  mechanical  parts  of  the 
microscope  can  therefore  be  best  discussed  under  the  different 
types  of  microscopes  applied  to  special  investigations. 

The  optical  components,  however,  need  a  few  words  in  order 
that  the  student  may  refresh  his  memory  relative  to  the  optics 
involved. 

Objectives  have  as  their  function  the  formation  of  an  enlarged 
real  image  of  the  object  placed  upon  the  stage  of  the  microscope. 
From  the  viewpoint  of  the  chemist,  their  construction  should  be 
such  as  to  keep  them  as  far  above  the  object  as  possible,  yet 
yield  an  image  of  as  great  an  area  of  the  object  as  can  be  ob- 
tained without  distortion  and  without  color  bands  or  fringes. 
In  addition,  they  should  possess  considerable  depth  of  focus. 

Objectives  are  commonly  designated  by  their  equivalent  focal 
length,  as,  for  example,  1  inch,  32  millimeters,  etc.,  the  numbers 
indicating  that  the  objective  will  produce  a  real  image  of  approxi- 
mately the  same  size  as  that  produced  by  a  simple  convex  lens 


2  ELEMENTARY   CHEMICAL   MICROSCOPY 

whose  principal  focus  lies  at  the  distance  marked  upon  the  ob- 
jective. 

In  a  similarly  constructed  series,  the  smaller  the  value  of  the 
equivalent  focus,  the  greater  will  be  the  magnifying  power  of  the 
objective.  A  few  manufacturers  still  arbitrarily  letter  or  number 
their  objectives.  In  such  cases  it  is  generally  the  rule  that  the 
earlier  in  the  alphabet  the  letter  or  the  smaller  the  number  in 
the  series  the  lower  the  magnifying  power. 

When  properly  focused  upon  a  preparation,  the  front  or  lowest 
lens  entering  into  the  construction  of  an  objective  is  usually 
nearer  to  the  preparation  (in  dry  objectives)  than  the  distance 
indicated  by  the  equivalent  focus.  This  distance  between  the 
front  combination  of  the  objective  and  the  preparation,  when  in 
focus,  is  known  as  the  working  distance  of  an  objective.  In  their 
selection  for  use  in  microchemical  analysis  the  working  distance 
becomes  one  of  the  most  important  considerations  affecting  the 
choice  of  the  objectives. 

The  construction  of  typical  microscope  objectives  is  shown 
diagrammatically  in  Figs.  17,  18  and  19,  page  44. 

All  objectives  are  corrected  to  a  greater  or  lesser  degree  for 
chromatic  aberration  (presence  of  colored  fringes  around  the 
images)  and  also  largely  for  spherical  aberration  (failure  to  yield 
a  flat  field  of  view).  When  the  spherical  aberration  is  so  corrected 
as  to  yield  an  especially  large  and  Jlal  field  the  objectives  are 
often  called  aplanalic  objectives.  Although  an  objective  may 
be  so  corrected  as  to  yield  a  flat  field,  images  of  objects  lying  near 
the  circumference  are  apt  to  be  hazy  or  indistinct,  the  result  of 
a  form  of  spherical  aberration  known  as  coma;  this  is  especially 
marked  in  high  power  objectives  and  requires  unusual  care  in 
construction  for  its  elimination.1 

In  all  ordinary  so-called  achromatic  objectives  the  corrections 
are  usually  such  as  to  bring  the  rays  of  two  spectral  colors  to  a 
focus.  In  such  lenses  the  optical  and  chemical  foci  may  lie  in 
different  planes  and  therefore  such  objectives  may  not  give 
really  good  results  if  employed  in  photomicrography;  for  this 
reason  specially  corrected  achromatic  lenses  called  photo- 
1  See  Spitta,  Microscopy,  London,  1909. 


OBJECTIVES  AND   OCULARS  3 

objectives  are  manufactured.  When  in  the  correction  for  chro- 
matic aberration  three  spectral  color  rays  are  brought  to  a  common 
focus  the  objectives  are  known  as  apochromatic  objectives.  In 
these  objectives  the  chemical  and  optical  foci  are  identical  and 
we  have  the  highest  grade  of  lenses  at  present  available.  Al- 
though in  apochromatic  objectives  rays  of  three  colors  are  brought 
to  a  correct  focus,  the  images  produced  by  these  three  sets  of 
rays  are  not  coincident  and  thus  yield  a  colored  fringe  or  halo 
at  the  edges  of  the  field.  This,  however,  is  eliminated  by  em- 
ploying slightly  over-corrected  eyepieces,  known  as  compensating 
eyepieces,  in  which  the  construction  is  such  as  to  neutralize,  or 
compensate  for,  the  errors  due  to  the  objectives.  Beautifully 
clear,  colorless  images  are  thus  obtained,  but  the  field  is  rarely  flat. 
Objectives  are  either  dry  or  immersion  according  as  they  are 
designed  to  be  used  with  air  or  with  some  liquid  between  the 
front  or  lower  lens  and  the  preparation.  High  power  dry  ob- 
jectives must  each  be  specially  adjusted  for  a  certain  definite 
thickness  of  cover  glass.  In  order  to  permit  some  freedom  of 
choice  in  cover  glasses  most  high  grade  high  power  dry  objectives 
are  adjustable  and  are  provided  with  a  movable  graduated  collar, 
permitting  the  regulation  of  the  objective  for  the  thickness  of  the 
cover  glass  used;  that  is,  a  part  of  the  combination  of  lenses 
making  up  the  objective  may  be  raised  or  lowered  in  the  mount- 
ing, thus  affording  a  correction  for  the  displacement  of  the  image 
brought  about  by  the  cover  glass.  By  consulting  the  diagram, 
Fig.  19,  page  44,  it  will  be  seen  that  by  turning  the  collar  C  the 
combination  of  lenses  L  will  be  displaced  and  their  distance 
from  the  combination  1/  will  either  be  increased  or  diminished. 
A  cover  glass  which  is  thicker  than  that  for  which  the  objective 
is  corrected  affects  the  image  in  the  same  manner  as  if  the  spheri- 
cal aberration  were  over-corrected,  while  on  the  other  hand  if 
too  thin  the  effect  produced  is  similar  to  that  of  under-correc- 
tion. In  the  first  case  the  focal  distance  of  the  objective  must 
be  increased,  and  in  the  second,  decreased.  This  is  accomplished 
by  turning  the  adjusting  collar  to  the  right  or  left,  as  the  case 
may  require,  or,  in  the  absence  of  such  a  device,  by  shortening 
or  lengthening  the  distance  between  the  eyepiece  and  the  objec- 


4  ELEMENTARY   CHEMICAL   MICROSCOPY 

tive,  shortening  for  cover  glasses  too  thick,  and  lengthening  for 
those  which  are  too  thin.  Fitting  into  the  body  tube  of  modern 
microscopes  is  a  tube  which  may  be  drawn  out  several  centi- 
meters. This  tube  is  known  as  the  draw-tube  and  is  graduated 
in  millimeters.  Objectives  are  commonly  corrected  (for  use  on 
the  usual  type  of  microscope)  for  a  tube  length  of  160  milli- 
meters. The  1 60-millimeter  mark  will  therefore  be  found  only 
when  the  draw-tube  is  pulled  out  a  short  distance.  This  position 
of  the  standard  mark  permits  lengthening  or  shortening  the  draw- 
tube,  and  thus  correcting  for  cover  glass  thickness  as  stated  above. 

In  addition  to  corrections  for  chromatic  and  spherical  aberra- 
tion at  least  two  other  factors  must  be  taken  into  account  in 
comparing,  or  choosing  between,  objectives  of  similar  equivalent 
focal  length.  These  are  the  angular  aperture  and  the  numerical 
aperture  of  the  objectives.  By  the  angular  aperture  of  an  ob- 
jective is  meant  the  "angle  contained,  in  each  case,  between 
the  most  diverging  rays  issuing  from  the  axial  point  of  an  object 
(i.e.,  a  point  in  the  object  situated  on  the  optic  axis  of  the  micro- 
scope), that  can  enter  the  objective  and  take  part  in  the  formation 
•of  an  image"  (Carpenter-Gage). 

This  angle  is  obviously  that  of  the  cone  of  light  rays  whose 
apex  lies  in  the  optic  axis  of  the  microscope  at  the  point  where 
the  axis  passes  through  the  plane  of  the  object  and  the  diameter 
of  whose  base  is  equivalent  to  the  opening  of  the  front  lens  com- 
bination of  the  objective. 

Dry  objectives  may  be  compared  with  each  other  with  refer- 
ence to  their  angular  aperture.  In  general  the  angular  aperture 
depends  largely  upon  the  diameter  of  the  front  combination  of 
the  objective,  and  usually  in  objectives  of  like  magnifying  power, 
the  greater  this  diameter  the  larger  will  be  the  angular  aperture 
and  the  wider  and  clearer  will  be  the  area  ox  field  covered.  It  is 
also  generally  true  that  the  shorter  the  equivalent  focus  of  the 
objective,  the  larger  its  angular  aperture  and  that  dry  objectives 
of  small  working  distance  usually  have  large  angular  apertures. 
It  is  obvious  that  in  dry  objectives  an  easy  comparison  of  the 
relative  areas  of  field  covered  is  afforded  by  a  consideration  of 
angular  apertures.     The  true  field  of  view  of  a  compound  mi- 


OBJECTIVES  AND   OCULARS  5 

croscope  is,  however,  controlled  by  the  ocular,  as  will  be  seen 
below. 

It  would  appear  at  first  sight  that  the  light-grasping  power 
of  an  objective  is  indicated  by  its  angular  aperture.  Such  is  not 
the  case,  for  Abbe  has  proved  that  in  comparing  objectives  as  to 
their  light-grasping  and  transmitting  power  it  is  the  sine  of  half 
the  angle  of  aperture  which  should  be  taken  into  account  and  not 
the  angular  aperture;  and  further,  that  since  objectives  are 
not  all  dry,  the  index  of  refraction  of  the  medium  between  the 
objective  and  the  object  must  necessarily  be  considered.  It  is 
therefore  now  conceded  that  the  light-grasping  and  transmitting 
power  of  an  objective  is  equal  to  the  refractive  index  of  the 
medium  in  which  the  objective  dips  multiplied  by  the  sine  of 
half  the  angle  of  aperture.  The  product  is  what  is  known  as 
the  Numerical  Aperture  and  is  expressed  N.A.  =  n  •  sin  n. 

If  the  above  formula  is  accepted  as  true  it  is  evident  that 
if  the  value  of  n  is  increased  the  numerical  aperture  will  likewise 
be  increased. 

The  light  rays  illuminating  an  object  by  transmission  through 
the  preparation  evidently  pass  from  a  denser  medium  (object) 
to  a  rarer  medium  (air),  and  following  the  law  of  refraction  are 
bent  away  from  the  perpendicular.  Hence  part  of  these  light 
rays  are  lost,  since  they  are  bent  so  far  that  they  cannot  enter 
the  small  front  lens  of  the  objective.  To  prevent  this  loss  and 
secure  a  brilliant  image  it  is  necessary,  according  to  the  formula 
N.A.  =  n  •  sin  n,  to  increase  the  value  of  n.  Therefore,  to 
obtain  very  high  powers,  the  substitution  of  some  liquid  for  air 
(n  =  1)  between  the  objective  and  the  preparation  becomes 
imperative  in  order  that  the  image  may  be  bright  and  distinct.1 

Objectives  permitting  the  use  of  a  liquid  in  this  manner 
are  known  as  immersion  objectives.  When  water  is  employed 
(n  =  1.33)  they  are  called  water  immersion,  and  when  an  oily 
liquid,  oil  immersion.  Usually  the  oil  consists  of  slightly  thick- 
ened oil  of  cedar  wood  {n  =  1.52),  and  since  the  refractive  index 
of  glass  object  slides  and  cover  glasses  is  approximately  1.52 

1  Abbe  found  that  the  brightness  of  the  image  varies  as  the  square  of  the  numer- 
ical aperture. 


ELEMENTARY  CHEMICAL  MICROSCOPY 


also,  such  objectives  are  more  commonly  designated  homog- 
enous immersion  objectives.  Alpha  mono-brom  naphthalene 
is  also  sometimes  used  as  an  immersion  fluid  (n  =  1.66)  and 
gives  us  the  highest  numerical  aperture  obtainable.1  Since  oil 
immersion  objectives  have  the  highest  numerical  apertures  they 
therefore  yield  the  brightest  and  the  clearest  images,  and  repre- 
sent the  highest  development  in  the  art  of  microscopic  objective 
manufacture. 

In  the  case  of  immersion  objectives  the  working  distances 
are  usually  greater  than  the  equivalent  foci. 

Variable  Objectives  are  so  constructed  that  the  distances  be- 
tween two  sets  of  component  lenses  may  be  changed  by  means  of 

a  graduated  collar,  permitting  a  wide  range 
in  the  magnifying  power  of  the  objective. 
A  single  objective  is  thus  made  to  do  the 
same  work  as  a  number  of  objectives  of 
fixed  system.  For  low  powers,  the  chemist 
will  find  an  objective  of  this  sort  an  ex- 
ceedingly great  convenience.  Fig.  1  shows 
a  variable  objective  as  manufactured  by 
Zeiss.  Its  range  of  magnification  lies  be- 
tween 29  and  43  diameters  and  its  free 
working  distance  between  the  limits  53 
millimeters  and  13  millimeters.  To  obtain 
a  similar  range  with  non-variable  objectives  requires  four  or 
five.  Variable  objectives  do  satisfactory  work  and  are  rela- 
tively inexpensive. 

A  measure  of  the  quality  of  an  objective  lies  in  its  ability  to 
make  clear  any  fine  and  delicate  details  of  structure.  It  is, 
therefore,  customary  to  speak  of  the  resolving  power  of  objectives 
and  express  this  attribute  in  terms  of  the  number  of  fine  lines  per 
unit  length  the  different  objectives  will  render  distinctly  visible, 

1  An  Abbe  condenser  of  the  commonly  purchased  form  has  as  its  maximum 
a  N.A.  of  1.20;  while  the  three  lens  condensers  of  the  highest  type  will  trans- 
mit rays  only  up  to  a  numerical  aperture  of  1.40.  Unless  therefore  a  special 
achromatic  condenser  is  available,  it  is  manifestly  useless  to  employ  alpha  mono- 
brom  naphthalene  immersion  objectives  since  only  a  part  of  the  full  aperture 
will  be  available. 


Fig.  1.     Zeiss  Variable 
Objective. 


OBJECTIVES   AND   OCULARS  7 

or,  in  other  words,  the  resolving  power  of  an  objective  can  be 
defined  as  the  minimum  distance  apart  two  lines  or  spots  may  be 
and  yet  appear  as  two  distinct  individuals.  The  resolving  power 
of  an  objective  is  dependent  upon  its  light  collecting  and  light 
transmitting  power;  this  in  turn  is  governed  by  the  numerical 
aperture  and  by  the  particular  wave  length  of  light  entering 
the  lens  system. 

From  the  viewpoint  of  the  physicist  the  resolving  power  of 

an  objective  can  be  expressed  as  equivalent  to  ,  where  X  is 

2  JN  .A.. 

the  wave  length  of  light.     This  is  based  upon  the  assumption 

that  the  illuminating  cone  of  light  completely  fills  the  aperture 

of  the  objective.     From  this  formula,  we  find  that,  theoretically, 

the  limit  of  resolution  will  be  attained  when  the  magnification 

of  an  objective  reaches  about  900. 

The  chemist  is  not  alone  interested  in  the  brightness  of  image 
and  the  resolving  power  of  an  objective,  but  he  is  vitally  con- 
cerned with  another  property,  namely,  the  ability  of  the  objective 
to  make  clear  objects  or  structures  in  more  than  one  plane. 
This  is  known  as  its  penetrating  power.  The  penetrating  power 
of  an  objective  has  been  shown  to  be  inversely  proportional  to 
the  numerical  aperture  and  to  vary  as  the  square  of  the  equiv- 
alent focus. 

Leaving  out  of  consideration  the  numerical  aperture,  it  is 
found  that  the  resolving  power  of  an  objective  is  inversely  pro- 
portional to  the  wave  length  of  light.  By  employing  light 
rays  of  very  short  wave  lengths  we  may  thus  obtain  exceptional 
resolution. 

In  the  above  consideration  it  has  been  assumed  that  the  illu- 
minating cone  of  light  completely  fills  the  aperture  of  the  objective. 
Nelson *  has  shown  that  in  practice  with  the  older  types  of 
objective  we  can  rarely  count  upon  more  than  three-fourths 
of  the  available  numerical  aperture.  More  modern  objectives 
perform  somewhat  better. 

In  comparing  objectives  as  to  their  ability  to  render  struc- 
tures clear  and  distinct  it  is  usual  to  do  so  by  computing  the 

1  J.  Roy.  Micro.  Soc,  1893,  15-17. 


8  ELEMENTARY  CHEMICAL  MICROSCOPY 

number  of  ruled  lines  to  the  inch  or  millimeter  each  one  will 
make  clearly  visible  (resolve).  To  obtain  these  values  the  recip- 
rocal of  the  above  given  standard  formula  must  be  taken.  Since, 
as  pointed  out,  we  cannot  obtain  the  theoretical  resolving  power 
in  practice  a  correction  coefficient  must  be  introduced  into  our 
formula.  Nelson  assigns  to  this  coefficient  the  value  1.3.  The 
practical  working  formulas  then  become : 1 

Available  resolving  power  = 


1.3  X 

/N  A 
Available  illuminating  power  = 


1.3  X 
L3X 


2 


Available  penetrating  power  =  -~~ 

N.A 


For  white  light  a  mean  value  may  be  assumed  to  be  X  =  5607 
(=  0.5607  n)  and  for  blue  light  X  =  4861  (=  0.4861  n). 

Advantage  has  been  taken  of  the  increased  resolving  power 
attainable  by  short  wave  lengths  in  the  application  of  ultra- 
violet light  (X  25oo±)  to  photomicrography.  In  this  way  a 
resolving  power  of  three  times  that  obtainable  with  red  light 
(X  7500 ±)  may  theoretically  be  obtained.  Since  ordinary  glass 
is  opaque  to  rays  below  X  3000,  it  is  essential  that  the  condenser, 
objectives,  oculars,  object  slides,  etc.,  be  made  of  quartz.  For 
similar  reasons  quartz  is  preferable  to  glass  in  all  ultramicroscopy, 
moreover,  most  glass  exhibits  a  marked  violet  fluorescence  under 
the  influence  of  ultraviolet  rays;  quartz  does  not. 

SELECTING   OBJECTIVES. 

It  is  evident  from  the  above  briefly  outlined  considerations, 
that  the  choice  of  an  objective  of  a  given  equivalent  focus  and 
magnification  must  depend  upon  the  nature  of  the  work  the 
objective  will  be  required  to  perform.  In  microchemical  analy- 
sis, because  of  the  rather  unusual  conditions  which  obtain,  ob- 
jectives must  be  selected  with  special  reference  to  long  working 
distance  and  great  depth  of  focus;  the  brightness  of  field  and  the 
resolving  power  necessarily  lost  are,  in  this  class  of  work,  of 

1  Nelson,  J.  Roy.  Micro.  Soc,  1906,  521. 


OBJECTIVES  AND  OCULARS  9 

little  importance,  since  only  low  powers  are  employed  and  the 
indices  of  refraction  of  objects  and  surrounding  medium  are 
generally  sufficiently  different  to  permit  an  easy  study  of  the 
preparations.  When  magnifications  of  from  300  to  500  are 
required  in  microchemical  examinations,  difficulty  will  be  expe- 
rienced in  obtaining  suitable  objectives  unless  the  prospective 
purchaser  stipulates  long  working  distances,  since  the  working 
distance  of  those  manufactured  for  the  use  of  biologists  is  far 
too  short  to  permit  their  application  to  the  study  of  uncovered 
and  therefore  thick  drops  of  liquid. 

For  the  study  of  objects  lying  in  a  single  plane,  for  polished 
surfaces,  rulings,  fine  etchings,  etc.,  in  which  sharpness  of  out- 
line and  delicacy  of  structure  or  tracery  are  present,  flatness  of 
field  and  high  numerical  aperture  are  essential.  Our  choice  is, 
consequently,  here  restricted  to  aplanatics  or  to  apochromatics, 
bearing  in  mind  the  fact  that  the  resolving  power  of  an  immersion 
objective,  where  applicable,  is  greater  than  that  of  a  dry  one. 

If,  on  the  other  hand,  the  investigation  to  be  conducted 
involves  much  photomicrographic  work,  photo-objectives,  apo- 
chromatics, or  better  still,  the  very  carefully  constructed  micro- 
planars,  microsummars,  or  microanastigmats,  should  be  selected. 
For  in  addition  to  the  fact  that  the  chemical  or  actinic  rays 
are  not  properly  brought  to  a  focus,  it  should  be  remembered 
that  ordinary  microscopic  objectives  are  corrected  for  a  fixed 
tube  length,  usually  160  millimeters,  while  in  the  case  of  photo- 
graphic work  the  distance  between  objective  and  plate  holder 
is  variable  and  in  all  cases  much  greater  than  the  standard  tube 
length. 

THE   CARE    OF   OBJECTIVES. 

Objectives  should  always  be  most  carefully  handled  and  pro- 
tected from  dust  and  vapors.  They  should  be  kept  dry  and 
clean  by  wiping  with  clean  new  lens  paper.1  Never  use  a 
piece  of  lens  paper  more  than  once,  nor  touch  the  lenses  of 
objectives  or  oculars  with  the  fingers  or  with  cloths. 

1  "Lens  paper"  is  a  soft  absorbent  tissue-like  paper  made  from  long  flexible 
fibers  expressly  for  cleaning  lenses. 


IO  ELEMENTARY   CHEMICAL  MICROSCOPY 

When  abrasives  are  employed  (as,  for  example,  in  metallo- 
graphic  work)  even  in  adjoining  rooms,  all  lenses  should  first  be 
blown  upon  (but  not  breathed  upon)  and  then  dusted  off  with 
a  very  soft  camel's  hair  brush  before  wiping  with  lens  paper, 
otherwise  serious  scratching  of  the  glass  will  sooner  or  later  result. 

Dust  on  the  back  lens  combination  of  the  objective  is  often 
responsible  for  great  loss  of  definition  and  greatly  reduces  the 
resolving  power  of  an  objective.  Dust  on  the  rear  lens  may 
easily  be  seen  by  removing  the  ocular,  illuminating  the  objec- 
tive to  its  full  capacity  and  looking  into  the  microscope  tube. 
Often  a  screen  of  ground  glass  placed  in  front  of  the  microscope 
mirror  renders  the  dust  particles  more  clearly  discernible. 

After  using  an  immersion  objective  immediately  wipe  off  the 
immersion  fluid  with  lens  paper,  then  if  the  fluid  is  oil,  wipe  the 
lens  with  lens  paper  moistened  with  xylene,  and  finally  wipe 
dry.  Never  use  alcohol  in  cleaning  objectives  or  any  part  of 
the  microscope.  Never  allow  an  objective  to  remain  moistened 
with  any  fluid  whatsoever  a  moment  longer  than  absolutely 
necessary. 

When  focusing  a  microscope  upon  a  preparation,  first  turn  the 
body  tube  down  by  means  of  the  coarse  adjustment  until  the 
objective  is  closer  to  the  preparation  than  is  indicated  by 
the  equivalent  focus  of  the  objective,  watching  carefully  with 
the  head  to  one  side  to  see  that  the  front  lens  is  not  forced 
against  the  slide.  Look  into  the  microscope  and  slowly  raise 
the  tube  by  the  coarse  adjustment  until  the  object  is  almost 
in  focus;  complete  the  adjustment  by  means  of  the  fine  adjust- 
ment. Never  focus  down  while  looking  into  the  instrument. 
Failure  to  observe  this  simple  rule  is  apt  to  lead  to  serious  loss 
and  considerable  expense. 

Never  change  from  one  objective  to  another  without  first 
making  sure  that  the  body  tube  has  been  raised  sufficiently  to 
allow  the  new  objective  to  be  slipped  into  place  without  injury 
to  the  preparation  on  the  stage  or  to  the  objective. 

Never  handle  objectives  or  oculars  or,  in  fact,  any  parts  of 
the  microscope  with  dirty,  greasy,  or  wet  fingers,  or  when  the 
hands  are  so  cold  as  to  incur  danger  of  dropping  the  apparatus. 


OBJECTIVES   AND   OCULARS  II 

Never  use  a  high  power  until  the  preparation  has  first  been 
examined  and  centered  with  a  low  one.  Remember  that  it  is 
possible  to  see  more  of  the  object  and  see  it  better  with  low 
powers  than  with  high  ones. 

Invariably  work  with  the  lowest  power  which  will  clearly 
define  the  preparation.  The  most  common  fault  of  the  beginner 
is  to  employ  too  high  a  magnification. 

Oculars.  —  The  function  of  the  ocular  or  eyepiece  of  a  com- 
pound microscope  is  to  magnify  the  real  inverted  image  of  the 
object  formed  by  the  objective;  but  in  addition  to  this  the  usual*  \y^ 
type  of  ocular  employed  serves  as  a  collector  of  light  rays  and  I 
increases  the  brilliancy  of  the  image  and  therefore  of  the  useful' 
area  of  the  field  of  view. 

Eyepieces  are  of  two  types,  those  in  which  the  real  image  is 
formed  inside  the  lens  system  of  the  ocular,  and  those  in  which 
the  real  image  is  formed  outside  the  ocular.  The  former  are 
known  as  negative  or  Huygenian  eyepieces;  the  latter,  as  positive 
or  Ramsden  eyepieces. 

Oculars  are  designated  either  by  their  equivalent  focal  length, 
by  the  number  of  times  they  magnify  the  real  image  formed  by 
the  objective  or  by  arbitrary  numbers  or  letters  based  upon 
either  equivalent  focus  or  magnification.  The  shorter  the 
equivalent  focal  length  the  higher  the  magnification.  When 
designated  by  their  magnification  the  figures  with  which  they 
are  marked  indicate  the  number  of  times  the  real  image  is  mag- 
nified. 

In  all  ordinary  microscopic  work  negative  or  Huygens  oculars  f 
are  employed,  the  use  of  positive  or  Ramsden  oculars  being 
restricted   to  micrometer   eyepieces.     In   the   case  of  positive 
oculars  the  entire  lens  system  acts  as  a  magnifier. 

The  usual  type  of  negative  eyepiece  is  shown  in  section  in  Fig.  2, 
with  the  passage  of  the  light  rays  diagrammatically  indicated. 
It  will  be  seen  on  consulting  the  diagram  that  the  lower  or  field 
lens,  as  already  stated,  collects  the  light  rays  and  reduces  the 
size  of  the  image  formed  by  the  objective  and  is  thus,  optically, 
in  reality  a  part  of  the  objective  system ;  the  eye  lens  functions 
as  a  magnifier  of  the  image  formed  by  the  field  lens.     It  is  evident 


12 


ELEMENTARY   CHEMICAL  MICROSCOPY 


Diaphragm 


that  the  position  and  diameter  of  the  diaphragm  in  the  eyepiece 
greatly  influence  the  character  and  size  of  the  field  lens  image, 
and  are  thus  largely  responsible  for  the  area  of  the  field  of  the  mi- 
croscope, and  consequently  are  very  closely  associated  with  the 

resolving  power  of  the  optical 
combination  employed.  The 
light  rays  leaving  the  eye  lens 
are  concentrated  within  a  tiny 
circle,  known  as  the  eye-point, 
eye-circle,  Ramsden  disk,  or 
Ramsden  circle.  The  desig- 
nation "eye-point"  has  been 
given  to  this  smallest  bright 
spot  of  light,  since  it  is  the 
proper  position  for  the  pupil 
of  the  eye  when  looking  into 
the  microscope.  If  either 
above  or  below  the  eye-point, 
light  rays  are  lost  and  the 
image  is  less  bright  and  less 
clear.  The  diameter  of  the 
eye-point  is  dependent  upon 
the  numerical  aperture  of  the 
objective  and  the  magnifica- 
tion of  the  microscope.  It  will 
be  found  upon  measuring  the 

Path  of  Light  Rays  in  a  Negative    j.  r    j_t_  •     1 

t?      .  diameters   of    the   eye-circles 

Eyepiece.  J 

produced  by  different  oculars 
with  the  same  objective,  that  they  are  inversely  proportional 
to  the  magnification  obtained  and  that  with  different  objec- 
tives and  one  and  the  same  eyepiece,  the  diameter  of  the  eye- 
circle  varies  directly  as  the  numerical  aperture  of  the  objectives. 
The  value  of  the  numerical  aperture  in  any  consideration  of 
the  probable  performance  of  different  objectives  of  the  same 
equivalent  focus  has  already  been  alluded  to.  We  now  see  that 
there  is  a  close  relation  existing  between  numerical  aperture  and 
the  performance  of  the  ocular;  for  example,  of  several  objectives 


Fig. 


2. 


OBJECTIVES   AND   OCULARS  13 

of  approximately  the  same  equivalent  focus,  but  possessing 
different  numerical  apertures,  that  one  having  the  highest  aper- 
ture will  permit  the  employment  of  an  ocular  of  much  higher 
power  and  thus  yield  a  considerably  greater  magnification 
without  loss  of  detail. 

If  an  attempt  is  made  to  increase  the  ocular  magnification 
beyond  a  certain  limit  the  eye-point  becomes  so  small  that  the 
image  resulting  is  blurred  and  indistinct.  This  fact  must  be 
borne  in  mind  in  microchemical  examinations  where  high  mag- 
nifications must  often  be  brought  about  by  using  high  power 
oculars  with  low  power  objectives  of  long  working  distance. 

In  order  that  images  of  satisfactory  distinctness  and  sharpness 
of  detail  may  be  obtained,  the  optical  combination  for  work 
must  be  such  as  to  yield  an  eye-point  not  less  than  one  milli- 
meter in  diameter  nor  greater  than  the  diameter  of  the  pupil 
of  the  eye  of  the  observer.1  The  diameter  of  the  eye-point  and 
the  position  of  the  plane  in  which  it  lies  can  easily  be  ascer- 
tained by  holding  a  piece  of  thin  ground  glass  or  waxed  paper 
over  the  ocular,  shading  it  with  a  screen  or  with  the  hand  and 
raising  or  lowering  it  until  the  bright  circle  seen  upon  the  glass 
or  paper  attains  its  minimum  diameter. 

Oculars  to  be  used  on  the  chemical  microscope  should  have 
the  plane  of  the  eye-circle  at  such  a  distance  above  the  eye-lens 
as  to  permit  the  adjustment  of  drawing  or  other  prisms  to  the 
position  of  maximum  brightness  and  diameter  of  field. 

Compensating  or  Compensation  oculars  are  eyepieces  specially  [ 
designed  for  use  with  apochromatic  objectives.     They  are  so 
called  because  of  the  fact  that  they  aid  in  the  correcting  of  chro- 
matic aberration. 

Oculars  are  said  to  be  par-focal  when  they  are  so  constructed 
as  to  permit  their  interchange  on  the  microscope  without  dis- 
turbing the  focus  of  the  instrument.2 

Compensating  oculars  are  usually  par-focal. 

1  Wright,  F.  E.,  The  Methods  of  Petrographic  Microscopic  Research,  Bui.  158, 
Carnegie  Inst.     Washington,  ion,  p.  38. 

2  For  a  consideration  of  the  conditions  to  be  fulfilled  in  their  construction,  see 
Gage,  The  Microscope,  p.  47.     Tenth  ed. 


14  ELEMENTARY    CHEMICAL   MICROSCOPY 

Projection  Oculars,  as  their  name  implies,  are  used  in  photog- 
raphy or  with  the  projection  microscope.  Their  purpose  is 
the  projection  of  a  bright  and  clear  image  upon  a  screen  whose 
distance  from  the  ocular  may  be  varied.  This  is  accomplished 
by  having  the  eye  lens  of  the  ocular  movable  in  the  mount,  thus 
changing  the  distance  between  eye  lens  and  ocular  diaphragm. 

Goniometer  oculars  are  eyepieces  provided  with  cross-hairs 
and  graduated  circle.  They  are  used  for  the  measurement  of 
crystal  angles  and  may  be  substituted  for  a  rotating  graduated 
stage  and  thus  permit  angular  measurements  on  any  microscope 
whose  tube  they  fit. 

The  Care  of  Oculars.  —  In  general  the  suggestions  made  with 
respect  to  objectives  on  pages  9,  10  and  11  apply  with  equal 
force  to  eyepieces. 

To  remove  cross-haired  oculars  grasp  them  firmly  between 
the  fingers  by  the  milled  head  and  first  lift  them  free  from  any 
slot  into  which  a  stud  upon  them  may  fit,  then  remove  them  by 
a  screw  motion. 

Dust  on  the  ocular  lenses  may  be  located  by  raising  and 
turning  the  entire  ocular,  then  by  unscrewing  and  turning  first 
the  field  lens,  then  the  eye  lens.  If  both  lenses  are  clean  and 
the  objective  is  clean  yet  the  field  shows  specks  of  dirt  and  ap- 
pears blurred,  the  dust  and  dirt  will  be  found  to  be  on  the  disk 
Carrying  the  cross-hairs  or  micrometer  scale.  Exceeding  great 
care  is  required  in  cleaning  cross-hairs  and  micrometer  plates 
resting  upon  the  diaphragm  of  the  ocular  and  should  only  be 
undertaken  by  a  person  having  patience,  care  and  steady  nerves. 

Use  low  oculars  first  and  confine  the  work  whenever  possible 
to  medium  powers.  Have  recourse  to  high  power  oculars  only 
as  a  last  resort,  since  they  cut  down  the  light  to  such  an  ex- 
tent as  to  cause  fatigue  and  eye-strain. 

Always  look  into  a  microscope  with  both  eyes  open. 

In  the  study  of  flat  preparations  between  slides  and  cover 
glasses,  the  general  rule  is  to  obtain  the  proper  magnification 
chiefly  by  means  of  the  objective,  using  a  low  power  ocular. 
But  in  the  case  of  irregular  surfaces  or  curved  and  heaped  up 
drops  of  liquid,  the  reverse  is  essential  and  low  power  objectives 


OBJECTIVES  AND   OCULARS  1 5 

(having  long  free  working  distance)  and  high  oculars  must  be 
adopted.  The  latter  procedure  is  also  indicated  when  employ- 
ing dark  ground  illuminators  or  ultra-condensers,  namely,  in- 
crease the  magnification  by  the  ocular. 

Limit  of  Magnification.  —  A  consultation  of  the  tables  of 
magnification  given  in  the  catalogues  of  the  leading  makers  of 
microscopes  and  microscope  lenses  will  show  that  with  the  modern 
compound  microscope  employed  in  the  usual  manner  with  stock 
achromatic  objectives  and  Huygenian  oculars,  a  magnification 
as  high  as  1500  to  2000  may  be  obtained,  and  that  with  stock 
apochromatics  and  compensating  eyepieces  this  may  still  further 
be  increased  to  3000,  the  upper  limit  of  listed  combinations. 

Theoretically  there  is  no  limit  to  the  magnification  which 
may  be  obtained.  But  this  must  not  be  confused  with  resolving 
power  which  enables  us  to  see  things  clearly  and  permits  differ- 
entiating one  part  or  structure  from  another.  Great  magnifica- 
tion avails  us  nothing  if  the  image  be  blurred  and  irrecognizable. 
A  little  thought  will  show  that  there  must  be  a  limit  to  the 
resolving  power  practically  available  beyond  which  we  cannot  go. 

The  shortest  violet  rays  producing  the  effect  of  light  upon  the 
average  normal  human  eye  may  be  assumed  to  have  a  wave 
length  of  approximately  X  4000  (or  0.4  n)1.  It  has  been  shown 
that  under  ordinary  conditions  the  smallest  particle  which  will 
be  visible  as  a  black  spot  upon  a  light  ground  must  have  a  diam- 
eter equal  to  at  least  half  this  value  (Helmholtz-Abbe).  More- 
over, a  lens,  owing  to  diffraction,  yields  as  an  image  of  a  point,  a 
diffraction  disk  and  not  a  point.  The  final  image  may  be  con- 
sidered as  consisting  of  a  series  of  diffraction  disks  or  patterns, 
and  if  the  distances  between  bright  points  are  such  as  to  cause  an 
overlapping  of  the  resulting  disks  or  their  surrounding  circles, 
a  blurring  of  the  image  must  result.  Thus  we  are  limited,  in 
our  attempt  to  see  and  study  infinitely  small  particles,  by  the 
sensitiveness  of  the  human  eye,  on  the  one  hand,  which  cannot 
properly  respond  to  the  stimuli  of  very  short  wave  lengths,  and 
to  the  fact,  on  the  other  hand,  that  no  matter  how  great  the  mag- 

1  One  micron,  designated  by  the  Greek  letter  /i,  is  equivalent  to  one-thousandth 
of  a  millimeter  (0.001  mm.). 


16  ELEMENTARY  CHEMICAL  MICROSCOPY 

nification  employed  we  cannot  bring  about  a  separation  of  the 
overlapping  rings  of  the  diffraction  patterns.  The  result,  there- 
fore, must  be  at  the  best  a  vague,  blurred,  uninterpretable 
image  or  merely  a  diffraction  pattern. 

If,  therefore,  our  wave  theory  of  light  is  correct,  the  most 
minute  particle  which  we  may  hope  to  render  distinctly  visible 
by  our  compound  microscopes  by  transmitted  light  must  have 
dimensions  of  at  least  0.2  fx.  It  should  not  be  inferred,  however, 
that  the  existence  of  particles  many  times  smaller  cannot  be 
indicated,  for  an  invisible  particle  may  yield  a  large  diffraction 
pattern,  a  phenomenon  which  makes  ultra-microscopic  investi- 
gations possible;  but  we  must  bear  in  mind  that  in  the  case  of 
ultra-microscopic  particles  we  have  no  picture  or  image  of  their 
shape  or  structure  and  that  we  know  of  their  existence  simply 
through  the  light  diffracted  by  them  and  thus  have  passed  far 
beyond  the  range  of  the  resolving  power  of  our  lenses.  Although 
it  is  true  that  the  limit  of  resolving  power,  0.2  n,  has  been  seri- 
ously questioned  by  men  of  recognized  authority,  it  may  be 
accepted  as  beyond  dispute  that  a  moderately  skillful  micros- 
copist  cannot  hope  in  practical  work  to  carry  the  resolving 
power  of  his  instrument  beyond  this  limit. 

In  ordinary  work  a  magnification  of  from  750  to  900  diameters 
is  the  upper  limit  of  true  usefulness  in  the  study  of  details  of 
structure.  Above  this  point  the  worker  must  be  an  exceptionally 
keen  and  skillful  observer  in  order  that  he  may  properly  interpret 
the  appearances  seen  in  the  images  formed. 

It  is  best,  therefore,  to  make  it  a  rule  to  work  with  low  mag- 
nifications. 


CHAPTER   II. 
MICROSCOPES   FOR  USE   IN   CHEMICAL  LABORATORIES. 

The  problems  which  the  chemist  is  called  upon  to  solve  where 
the  microscope  is  of  great  value,  if  not  actually  essential,  are 
so  diverse  in  their  nature  and  the  materials  to  be  examined  so 
varied  in  size,  outward  form,  structure  and  composition  that  it 
is  safe  to  say  that  no  single  instrument  will  ever  be  constructed 
which  will  meet  all  requirements  and  fulfill  all  conditions.  Before 
deciding  upon  any  given  style  or  model  of  instrument  the  in- 
tending purchaser  should,  therefore,  first  carefully  consider  the 
kind  of  work  his  instrument  will  most  frequently  be  called  upon 
to  perform. 

A  microscope  for  microchemical  analysis  and  applicable  to 
the  ordinary  problems  arising  in  the  chemical  laboratory  should 
fulfill  the  following  requirements: 

i.  The  stand  should  be  substantially  built  so  as  to  be  easily 
and  safely  carried  about.  It  should  permit  the  attachment  of 
the  usually  employed  accessories,  such  as  a  mechanical  stage, 
Abbe  condenser,  camera  lucida,  polarizing  apparatus,  etc.  A 
hinged  pillar  allowing  the  inclination  of  the  microscope  is  a 
valuable  feature  and  a  great  convenience.  In  a  vertical  position 
for  work  the  stand  should  be  low  enough  to  permit  observations 
being  made  in  comfort,  without  the  necessity  of  having  either 
specially  high  stools  or  low  tables.  It  is  desirable  that  the  in- 
strument be  entirely  finished  in  black  and  have  as  few  bright 
reflecting  surfaces  as  possible. 

2.  There  should  be  coarse  adjustment  by  diagonal  rack  and 
pinion  of  as  great  range  as  possible.  When  the  movement  of 
the  rack  is  short  the  usefulness  of  the  microscope  is  greatly 
restricted,  since  low  powers  cannot  then  be  used  with  thick  ob- 
jects. A  sensitive  fine  adjustment  is  also  an  essential,  and  if  the 
fine  adjustment  is  provided  with  micrometer  screw  and  gradu- 

17 


18  ELEMENTARY  CHEMICAL  MICROSCOPY 

ated  head,  micrometric  measurements  of  thickness  are  possible, 
and  refractrometric  determinations  are  simplified. 

3.  The  body-tube  carrying  the  objective  and  eyepiece  should 
be  of  sufficient  diameter  to  permit  the  microscope  being  used 
for  photography,  and  it  should  be  provided  with  an  inner  grad- 
uated draw- tube  whose  lower  end  is  tapped  with  standard  or 
universal  thread  for  the  attachment  of  very  low  power  objectives 
or  of  amplifiers. 

4.  The  stage  should  be  circular,  rotating  and  provided  with 
centering  screws  with  small  milled  heads.  The  circumference 
of  the  stage  should  be  graduated  in  degrees  and  the  surface 
covered  with  hard  rubber.  The  stage  must  be  constructed  in 
such  a  manner  as  to  be  easily  removed  by  simply  loosening  the 
centering  screws,  in  order  that  thick  objects  may  be  examined, 
various  heating  devices  employed,  and  opaque  objects  to  be 
studied  by  means  of  vertical  illuminators  may  be  brought  into 
focus  on  the  substage  without  interfering  with  the  proper  ad- 
justment of  radiant  or  illuminator. 

5.  The  substage  should  consist  of  a  simple  ring,  raised  and 
lowered  by  screw  or  rack  and  pinion,  and  must  permit  of  being 
swung  to  one  side  from  under  the  stage.  This  ring  carries 
condenser,  polarizer,  auxiliary  stage,  heating  devices,  etc.  The 
ring  should  be  tapped  at  one  side  and  fitted  with  thumb-screw 
or  with  some  sort  of  locking  device  to  hold  firmly  in  place  the 
accessories  fitting  into  the  substage  ring.  The  substage  ring 
should  also  be  provided  with  a  slot  or  other  contrivance  for 
lining  up  the  polarizer.  If  the  microscope  is  to  be  used  chiefly 
for  observations  at  high  temperatures,  polarization  by  reflection 
is  best. 

6.  It  is  essential  that  the  microscope  be  fitted  with  attach- 
ments for  study  with  polarized  light  including  converging  as 
well  as  plane.  For  all  ordinary  problems,  the  best  system  ap- 
pears to  be  rotating  prisms  of  the  type  of  the  Nicol  prism,  one 
placed  below  the  stage,  the  other  above  the  objective.  One  or 
both  of  these  polarizing  prisms  should  be  mounted  so  as  to  rotate 
and  be  provided  with  graduated  circles.  It  will  be  found  to  be 
a  great  convenience  if  the  construction  is  such  that  when  polarizer 


MICROSCOPES   FOR   USE   IN   CHEMICAL  LABORATORIES       19" 

and  analyzer  are  in  their  proper  places  the  planes  of  vibration 
of  these  prisms  will  be  crossed  without  the  necessity  of  experi- 
mental adjustment. 

7.  The  instrument  must  be  provided  with  a  mirror,  plane  on 
one  side,  concave  on  the  other,  of  as  large  diameter  as  possible, 
which  permits  turning  over  from  plane  to  concave  side  when 
the  microscope  is  in  a  vertical  position  without  the  necessity  of 
tipping  the  pillar.  The  mirror  should  be  mounted  on  a  swing- 
ing bar  to  provide  very  oblique  light  and  it  is  desirable  that  the 
bar  have  an  extension  arm  in  order  that  the  mirror  may  be  swung 
to  give  oblique  light  above  the  stage. 

8.  At  least  two  of  the  oculars  (a  high  power  and  a  low  power) 
must  be  fitted  with  cross-hairs  and  stud  fitting  into  a  notch  or 
slot  in  the  upper  end  of  the  draw-tube. 

9.  The  objectives  should  be  of  exceptionally  long  working 
distance  and  in  combination  with  the  eyepieces  should  yield  a 
magnification  of  from  15  or  20  diameters  to  300  or  350  diameters 
for  ordinary  work. 

10.  The  instrument  should  be  of  as  simple  construction  as 
possible  and  should  permit  the  easy  and  inexpensive  replace- 
ment of  parts  damaged  through  accident. 

TYPES    OF   MICROSCOPES    FOR    MICROCHEMICAL    INVESTIGA- 
TIONS. 

Instruments  for  General  Use.  —  A  microscope  which  con- 
forms very  closely  to  the  specifications  given  above  is  shown 
in  its  latest  model  in  Fig.  3.  This  instrument  has  been  con- 
structed after  specifications  of  the  author1  to  meet  most  of 
the  problems  arising  in  chemical  laboratories  in  which  a  micro- 
scope may  be  employed.  In  this  model  an  attempt  has  been 
made  to  provide  as  compact  an  instrument  as  possible,  having 
an  exceptionally  great  distance  between  the  optic  axis  and  the 
arm,  thus  providing  sufficient  manipulative  space  for  large  ob- 
jects, cells,  etc.;  the  range  of  the  body  tube  is  also  sufficient  to 
permit  even  very  low  powers  to  be  used  with  vertical  illumina- 

1  Chamot,  J.  Applied  Micros.,  2  (1899)  502.  Manufactured  by  the  Bausch 
&  Lomb  Optical  Co.,  Rochester,  N.  Y. 


20 


ELEMENTARY   CHEMICAL  MICROSCOPY 


Fig.  3.     Simple  Polarizing  Microscope  for  Chemical  Microscopy. 


MICROSCOPES   FOR   USE   IN   CHEMICAL   LABORATORIES       21 

tors,  while  the  range  of  the  substage  screw  is  long  enough  to 
permit  focusing  the  substage  ring  with  auxiliary  stage  attached 
in  metallographic  work,  thus  keeping  the  body  tube  with  an 
illuminator  in  line  with  the  radiant. 

The  milled  heads  of  the  stage  centering  screws  have  been  made 
much  smaller  and  shorter  than  usual  in  order  that  they  may 
interfere  less  with  manipulations  on  the  stage  and  be  less  subject 
to  displacement. 

The  revolving  stage  with  circle  graduated  into  degrees  is 
removable  by  merely  unscrewing  the  centering  screws,  and  then 
lifting  out  the  stage.  This  permits  inserting  into  the  substage 
ring  an  auxiliary  stage  for  use  with  thick  objects,  or  opaque 
objects,  to  be  studied  with  a  vertical  illuminator  (see  Fig.  41, 
page  88),  or  when  preparations  are  to  be  heated  with  a  tiny 
flame. 

The  polarizer  PO  consists  of  a  Nicol  prism  set  in  a  rotating 
mounting  graduated  into  degrees.  A  stud  in  the  fixed  part  of  the 
mounting  fits  into  a  slot  in  the  substage  ring,  thus  insuring  that 
the  polarizer  mounting  is  always  in  the  same  relative  position. 
The  analyzer,  PA,  a  Thompson  prism,  fits  over  the  eyepiece, 
rotates,  and  is  provided  with  a  graduated  circle.  In  the  mount- 
ing of  the  prism  provision  is  made  for  adjustment  in  a  vertical 
direction  so  as  to  ensure  a  wide  field  of  view  with  all  oculars. 
A  slot  in  the  collar  in  which  the  analyzer  revolves  engages  a 
stud  St  on  the  draw-tube  of  the  instrument.  The  draw-tube 
itself  moves  vertically  only,  thus  if  the  polarizer  and  analyzer  be 
properly  inserted  and  their  graduated  circles  set  at  zero,  the 
prisms  are  crossed  without  further  adjustment.  The  placing  of 
the  analyzer  over  the  eyepiece  in  a  microscope  for  microchem- 
ical  analysis  will  be  found  to  be  much  safer  than  the  more  con- 
venient mounting  sliding  into  the  body  tube,  as  in  petrographic 
instruments.  When  the  instrument  is  to  be  much  used  in  the 
microscopy  of  foods  a  supplementary  polarizer  may  be  obtained 
which  fits  into  the  ring  below  the  Abbe  condenser,  thus  allowing 
the  prism  to  be  swung  quickly  aside  without  interfering  with  the 
illuminating  devices. 

Instruments  made  by  other  firms  for  chemical  microscopy 


22 


ELEMENTARY  CHEMICAL  MICROSCOPY 


differ  but  little  from  that  shown  in  the  illustration.     It  has, 
therefore,  been  thought  unnecessary  to  picture  them  here. 

Microscopes  for  Special  Purposes.  —  When  large  samples  of 
powdered  material  are  to  be  investigated,  as  in  the  examination 
of  dry,  powdered  or  granulated  foods,  drugs,  etc.,  for  adulter- 
ation, a  microscope  with  large  stage  of  the  type  shown  in  Fig.  4 


Fig. 


Microscope  with  Large  Stage  for  the  Rapid  Examination  of  Powdered 

Material. 


is  of  great  assistance.1  The  material  is  thinly  spread  out  upon 
the  plate  glass  stage,  and  the  microscope  is  made  to  pass  by 
means  of  the  screws  S  and  R  over  the  entire  area  covered  by 
the  material.  A  very  low  power  L  is  first  employed  until  some 
particle  is  found,  needing  to  be  studied  more  carefully.  The  par- 
ticle is  centered  under  the  lens,  L  is  then  removed  and  the  com- 
pound microscope  M  slipped  in  place  in  the  same  slot  previously 
occupied  by  L.  The  particle  in  question  now  falls  under  the 
compound  microscope.  This  type  of  microscope  primarily  in- 
tended for  the  examination  of  large  sections  of  the  brain  will 

1  Made  by  E.  Leitz,  Wetzlar  and  also  by  Nachet  et  Fils,  Paris. 


MICROSCOPES   FOR   USE  IN   CHEMICAL   LABORATORIES       23 

be  found  a  great  saver  of  time,  labor  and  material.  Its  appli- 
cations are  many.  In  laboratory  work  involving  the  study  of 
plates  of  bacterial  cultures  it  will  be  found  to  be  far  superior 
to  microscopes  of  the  ordinary  type,  since  plates  of  large  size 
may  be  examined  at  any  point  within  their  areas. 

The  compound  microscope  is  provided  with  rack  and  pinion 
coarse  adjustment  and  with  a  quick  acting  screw  adapter  F 
fitted  to  the  end  of  the  body  tube  for  fine  adjustment. 

Comparison  Microscopes.  —  It  not  infrequently  happens  that 
it  is  found  desirable  to  carefully  compare  two  preparations  or 
two  different  samples.  This  is  especially  true  in  quantitative 
microscopy.  With  ordinary  microscopes  it  is  necessary  to  place 
first  one  sample,  then  the  other,  under  the  microscope,  make 
drawings,  measurements  and  take  mental  note  of  the  appearance 
of  each  preparation  in  turn  and  then  compare  the  mental  pictures 
by  the  aid  of  the  data  at  hand.  This  process  is  not  easy,  and  the 
results  not  always  trustworthy  even  in  the  hands  of  an  expert 
without  long  and  exceptionally  thorough  studies.  Photomicrog- 
raphy offers  a  fair  solution  but  here  again  the  time  required 
and  the  additional  manipulations  necessitated  prevent  its  general 
application. 

This  need  of  some  device  whereby  quick  and  rapid  compari- 
sons might  be  possible  has  long  been  felt,  but  no  suitable  instru- 
ments were  placed  upon  the  market  until  very  recently.  These 
new  instruments  have  received  the  name  Comparison  Micro- 
scopes. They  are  so  constructed  that  the  images  formed  by 
two  different  optical  systems  are  brought  into  juxtaposition, 
so  that  the  observer  is  able  to  simultaneously  see  the  images  of 
two  different  objects. 

As  long  ago  as  1885,  Inostranzeff1  employed  what  he  desig- 
nated as  a  comparison  chamber,  consisting  of  two  sets  of  totally 
reflecting  prisms  so  mounted  in  a  rectangular  chamber  as  to 
reflect,  into  a  single  eyepiece,  the  images  of  half  the  field  of  each 
of  two  microscopes. 

Two  years  later  Van  Heurck2  improved  the  Inostranzeff  in- 

1  Jahrb.  f.  Min.,  2  (1885),  94;     J.  Roy.  Micros.  Soc,  1886,  507. 

2  Van  Heurck,  J.  Roy.  Micros.  Soc,  1887,  463. 


24 


ELEMENTARY   CHEMICAL   MICROSCOPY 


strument  by  a  different  arrangement  of  prisms.  This  latter 
type  has  again  been  revived  by  the  Bausch  and  Lomb  Optical 
Company  in  191 2,  and  by  E.  Leitz  in  1914. 

A  somewhat  similar  comparing  device,  consisting  of  two 
totally  reflecting  prisms,  was  proposed  by  Ewell1  and  employed 
by  him  as  a  colorimeter.  The  Van  Heurck  comparison  eyepiece, 
Fig.  5,  as  constructed  by  Bausch  and  Lomb  consists  of  a  rec- 
tangular cell  provided  on  the  lower  side  with  two  orifices  and 


■ 

1 
1 

X 

,,,, '  .:■",".,"  — ~ Ta 


....■  ■■■'".  ■■.'■■■  .'.  '■  "".".  '  ".^'7 


Fig.  5.     The  Bausch  and  Lomb  Comparison  Eyepiece. 

with  tubes  T1  and  T2  of  the  same  diameter  as  ordinary  oculars, 
and  at  such  a  distance  apart  as  to  permit  their  simultaneous 
insertion  into  the  tubes  of  two  microscopes  placed  side  by  side. 
Midway  between  these  tubes  on  the  top  of  the  cell  is  an  opening 
with  a  tube  into  which  slides  a  Ramsden  eyepiece  O.  Above 
the  tubes  T1,  T2  are  placed  totally  reflecting  prisms   P1,   P2, 


Ewell,  J.  Roy.  Micros.  Soc,  1910,  14. 


MICROSCOPES   FOR   USE  IN   CHEMICAL  LABORATORIES       25 

which  reflect  the  images,  formed  by  the  objectives  of  the  micro- 
scope, into  the  rectangular  prisms  R1,  R2,  situated  just  below 
the  ocular  O.  The  prisms  R1,  R2  consist  of  rectangular  pieces 
of  glass  cut  through  diagonally  and  cemented  together,  the 
inclination  of  the  cut  surfaces  being  parallel  to  the  reflecting 
surfaces  of  P1,  P2,  respectively.  Upon  looking  into  the  ocular  O 
the  field  is  seen  to  be  divided  into  an  upper  and  a  lower  part  by  a 
line  passing  from  left  to  right.  It  is  obvious  that  the  image  of 
half  the  field  of  one  microscope  will  be  seen  in  one  of  the  halves 
of  the  ocular,  while  the  other  half  of  the  ocular  will  exhibit  half 
the  field  of  the  other  microscope.  In  order  to  facilitate  focusing 
the  microscopes  the  tube  T1  is  of  such  diameter  as  to  fit  snugly 
into  the  tube  of  one  of  the  microscopes,  while  the  tube  T2  is  of 
less  diameter  and  hence  fits  loosely.  The  microscope  carrying 
T1  is  therefore  focused  first.  Objects  to  be  carefully  compared 
by  means  of  this  instrument  must  necessarily  lie  in  the  same 
plane,  otherwise  the  magnification  in  one  half-field  will  be  greater 
than  in  the  other.  Where  slight  variations  in  magnification 
can  be  neglected,  the  thicker  preparation  is  placed  upon  the 
stage  of  the  microscope  carrying  the  tight  tube  of  the  comparison 
eyepiece,  or  if  chemical  microscopes  (Fig.  3,  page  20)  are  em- 
ployed, one  or  both  preparations  may  be  supported  upon  the 
auxiliary  stage  and  turned  down  until  the  upper  surfaces  of  the 
two  preparations  lie  in  the  same  plane.  This,  however,  is  only 
possible  when  no  substage  condenser  need  be  employed. 

Comparison  microscopes  proper  are  of  two  different  types, 
either  they  have  a  single  eyepiece  and  make  use  of  reflecting 
prisms  or  they  consist  of  two  microscopes  with  two  eyepieces, 
the  observer  using  both  eyes. 

The  Leitz1  comparison  microscope,  Fig.  6,  consists  of  two 
microscope  tubes  A,  B  attached  to  a  single  pillar  P  movable  by 
rack  and  pinion.  A  single  stage  S  is  provided  with  two  open- 
ings, one  for  each  microscope  tube.  Under  each  stage  opening 
is  placed  an  Abbe  condenser  with  iris  diaphragm  and  rings 
for  stops,  or  for  blue,  green  or  ground  glass.  Each  condenser  is 
illuminated  by  means  of  a  separate  mirror  on  a  swinging  bar  and 

1  Manufactured  by  E.  Leitz,  Wetzlar,  Germany. 


26 


ELEMENTARY   CHEMICAL   MICROSCOPY 


is  adjustable  up  and  down  by  a  friction  collar.     To  the  upper 
end  of  each  microscope  tube  is  attached  a  large  chamber  C,  C1 

containing  reflecting  erecting 
prisms.  Above  the  cham- 
bers are  the  oculars  E,  E1, 
provided  with  sliding  dia- 
phragms D1,  D2.  The  prism 
chambers  are  so  constructed 
as  to  rotate  through  a  small 
arc  in  the  directions  of  the 
arrows,  thus  bringing  the 
eyepieces  nearer  together  or 
farther  apart  for  adjustment 
of  the  proper  pupillary  dis- 
tance of  the  observer.  The 
upper  half  of  each  eyepiece 
can  also  be  rotated  so  that 
when  the  diaphragms  D1,  D2 
are  inserted  to  cut  off  half  the 
field  in  each  ocular,  they  may 
be  turned  until  the  diam- 
eters of  each  half  field  are 
parallel  or  coincident.  After 
turning  through  the  proper 
arc  the  thumb  screws  T1,  T2 
are  tightened  to  prevent  the 
adjustment  from  changing. 
By  proper  manipulation  of 
the  sliding  diaphragms,  the 
observer    looking    into    the 


Fig.  6.     The  Leitz  Comparison  Microscope. 


instrument  with  an  eye  above  each  ocular  sees  half  the  field 
from  one  preparation  and  half  from  the  other  in  close  juxtaposi- 
tion. A  very  rapid  yet  critical  comparison  of  one  preparation 
with  another  is  thus  easily  accomplished.  Or  D1,  D2  may  be  so 
placed  as  to  cut  out  the  field  of  either  tube,  or  if  both  are  pushed 
in  as  far  as  they  will  go  the  fields  will  be  superimposed,  and  the 
symmetry  of  two  objects  may  be  compared. 


MICROSCOPES   FOR   USE   IN   CHEMICAL   LABORATORIES       27 

The  coarse  adjustment  R  by  rack  and  pinion  serves  to  roughly 
focus  both  tubes  at  once;  then  each  objective  is  focused  sepa- 
rately by  means  of  the  fine  adjustment  screw  collars  F,  F1  just 
above  the  objectives.  That  really  satisfactory  results  may  be 
obtained  it  is  essential  that  both  the  sets  of  eyepieces  and  objec- 
tives shall  be  paired,  i.e.,  shall  have  been  constructed  for  use 
with  a  comparison  microscope  and  be  exactly  equivalent  in  all 
properties.  The  fields  are  flat,  brilliant,  and  with  careful  illu- 
mination and  adjustment  and  a  little  practice  most  excellent 
results  can  be  obtained.  The  instrument  is  adapted  to  all 
problems  involving  an  exact  comparison  of  size,  structure  or 
symmetry  of  microscopic  objects,  especially  where  the  structure 
is  so  intricate  as  to  render  comparison  and  interpretation  with 
the  ordinary  single  compound  microscope  exceptionally  difficult 
without  recourse  to  photography.  The  value  of  the  instrument 
in  all  problems  of  forensic  chemical  microscopy  is  evident. 

A  second  type  of  comparison  microscope1  is  provided  with  a 
single  eyepiece  only,  the  field  being  divided  into  halves.  As  in 
the  previously  described  instrument,  two  microscope  tubes  are 
attached  to  a  single  pillar  and  both  focused  together  by  rack 
pinion.  Attached  to  the  tubes  is  a  rectangular  closed  chamber 
of  the  Inostranzeff  type  provided  with  two  sets  of  totally  reflect- 
in  prisms,  thus  yielding  to  a  single  eyepiece  half  the  field  of 
view  of  each  microscope.  By  means  of  a  knob  in  the  side  of 
the  chamber  one  set  of  prisms  may  be  shifted  at  will  so  as  to  cut 
off  the  field  of  one  instrument. 

In  addition  to  a  single  fine  adjustment,  simultaneously  affect- 
ing both  microscopes,  each  tube  is  provided  with  independent 
fine  adjustment  collars  just  above  the  objectives.  A  single 
stage  with  two  openings  carries  two  substages,  each  with  an 
Abbe  condenser  and  with  a  mirror.  The  instrument  may  be 
employed  with  polarized  light,  thus  affording  exceptional 
opportunities  for  exact  comparisons  in  the  search  for  food 
adulterants  and  in  microchemical  analysis.  Since  in  this  in- 
strument we  have  a  single  ocular  yielding  a  divided  field,  it  is 
possible  to  obtain  photomicrographs,  half  the  area  of  the  circle 

1  W.  and  H.  Seibert,  Wetzlar,  Germany.     Thorner,  Chem.  Ztg.,  36,  781. 


28  ELEMENTARY   CHEMICAL  MICROSCOPY 

in  the  negative  obtained  being  the  image  of  one  preparation,  the 
other  half  that  of  the  second  preparation.  This  instrument 
consists  essentially  of  a  stand  similar  to  the  Leitz  with  the 
microscope  tubes  joined  by  a  prism  chamber  and  therefore  no 
illustration  of  its  construction  is  necessary. 

Photomicrographs  and  polarization  studies  are  of  course  also 
possible  with  the  comparison  eyepiece  described  above. 

When  two  microscopes  are  available  the  comparison  eyepiece 
will  be  found  to  perform  all  the  work  which  may  be  accomplished 
by  means  of  instruments  of  the  Seibert  type  and  will  entail 
little  additional  expense  to  the  equipment  of  the  microchemical 
laboratory. 

Comparison  microscopes  are  almost  indispensable  when  fre- 
quent comparisons  must  be  made  between  unknown  and  known 
or  standard  preparations,  or  when  rapid  approximate  quantita- 
tive results  are  required.  These  instruments  and  the  simple 
polarizing  compound  microscope  may  be  said  to  be  the  only 
ones  for  what  can  be  called  general  use  in  the  chemical  laboratory. 

Special  microscopes  for  micrometric  purposes,  such  as  read- 
ing scales,  determinations  of  the  positions  of  lines  in  the  photo- 
graphs of  spectra,  or  measuring  the  diameter  of  depressions 
produced  in  testing  for  hardness  by  the  Brinell  method,  will 
be  found  described  in  Chapter  VII,  page  147;  microscopes  for 
the  study  of  ultramicroscopic  particles  in  Chapter  IV,  page  54, 
while  the  special  types  of  instrument  for  the  examination  of 
metallurgical  products  and  large  castings  are  taken  up  in  detail 
in  Chapter  V. 

For  the  investigation  of  molten  material,  liquid  crystals,  etc., 
microscopes  of  special  construction  have  in  recent  years  been 
placed  upon  the  market.  Most  of  these  have  followed  the  de- 
signs of  O.  Lehmann  and  comprise  a  great  variety  of  forms.1 
One  of  the  simplest  of  these  is  shown  in  Fig.  7.  In  this  instru- 
ment polarized  light  (see  Chapter  VIII)  is  obtained  by  reflection 
instead  of  by  the  usual  manner  by  means  of  a  Nicol  prism,  in 
order  to  permit  swinging  the  tiny  Bunsen  burner  B  below  the 
stage.     The  light  rays  reflected  from  P  and  R  are  polarized  and 

1  See  Lehmann,  Das  Kristallisationsmikroskop,  Braunschweig,  19 10. 


MICROSCOPES   FOR   USE  IN   CHEMICAL  LABORATORIES       20 

are  sent  through  the  preparation  upon  the  stage  by  means  of 
the  mirror  M.  The  analyzer  consists  of  a  prism  sliding  in  and 
out  of  the  microscope  tube  at  A.     In  the  illustration  the  dotted 


Fig.  7.  Simple  Form  of  Hot-stage  Microscope.  Polarized  Light  is  obtained  by- 
Reflection  from  the  Plates  P  and  R  and  the  Mirror  M  as  indicated  by  the 
Dotted  Arrows.  A  =  Analyzer.  B  =  Small  Gas  Burner  which  swings  under 
the  Stage  Opening. 

lines  indicate  the  approximate  direction  of  the  light  rays  used 
to  illuminate  the  object.  When  moderate  temperatures  are 
necessary  the  objective  must  be  cooled  by  means  of  a  blast 
of  air  directed  upon  the  lower  lens,  and  when  high  temperatures 
are  employed  the  objective  must  be  water-jacketed. 


CHAPTER  III. 
ILLUMINATION   OF   OBJECTS;   ILLUMINATING   DEVICES. 

Illumination  and  Illuminating  Devices.  —  Of  even  greater 
importance  than  the  selection  of  the  correct  combination  of 
objective  and  ocular  for  the  study  of  a  preparation  is  the  matter 
of  proper  illumination.  The  earlier  in  his  work  the  student 
appreciates  the  importance  of  illumination  and  the  more  thought 
and  care  he  expends  upon  this  phase  of  microscopic  methods, 
the  fewer  errors  he  will  make  and  the  more  easily  will  he  accom- 
plish the  objects  of  his  investigations. 

For  convenience  of  discussion  the  modes  of  illuminating 
objects  for  microscopic  study  may  be  grouped  under  the  follow- 
ing heads: 

a.  Transmitted  axial  light. 

b.  Transmitted  oblique  light. 

c.  Reflected  axial  light. 

d.  Reflected  oblique  light. 

e.  Oblique  dark  ground  illumination. 

/.  " Orthogonal  illumination"  (Siedentopf  Slit  Ultrami- 
croscope). 

g.  Differential  color  illumination. 

h.  By  means  of  ultraviolet  light,  thus  causing  certain  sub- 
stances to  become  fluorescent. 

a.  Transmitted  Axial  Light  obtained  by  means  of  the  mirrors 
with  or  without  a  condenser  may  be  said  to  be  the  usual  or  most 
frequently  employed  method  of  illuminating  transparent  and 
translucent  objects.  With  low  power  objectives  and  objects  of 
coarse  structure  no  condenser  is  necessary,  but  when  the  object 
to  be  studied  presents  a  fine  structure  and  delicacy  of  tracery 
and  when  its  refractive  index  lies  close  to  that  of  the  mounting 
medium,  structural  studies  become  difficult,  if  not  impossible, 

3° 


ILLUMINATION  OF  OBJECTS;   ILLUMINATING   DEVICES       3 1 


without  moderately  high  powers  and  some  form  of  substage 
condenser.  It  is  therefore  a  safe  rule  to  always  employ  a  sub- 
stage  condenser  unless  exceptionally  low  powers  are  to  be  used; 
this  of  course  does  not  apply  to  problems  involving  examinations 
with  polarized  light. 

b.  Transmitted  Oblique  Light  is  essential  for  the  proper 
interpretation  of  appearances  under  the  microscope  of  objects 
whose  upper  and  lower  surfaces  are  so  placed  as  to  lead  to  serious 
confusion  if  axial  light  is  alone  employed.  Oblique  light  also 
aids  in  establishing  whether  the  liquid  medium  or  the  object 
immersed  in  it  has  the  higher  refractive  index.  The  value  of 
oblique  illumination  may  be  better  understood  by  referring  to 
the  diagram  shown  in  Fig.  8.  A  transparent  object  O  whose 
upper  and  lower  surfaces  are  identical 
and  perfectly  symmetrical,  is  shown 
in  section,  lying  upon  an  object  slide 
upon  the  stage,  with  perfectly  axial 
light  as  shown  by  the  arrows.  It  will 
be  obvious  that  even  very  careful  focus- 
ing will  fail  to  disclose  the  probable 
structure  of  the  lower  surface  and  that 
even  the  upper  surface  may  be  in 
doubt;  but  if  oblique  illumination  be 
employed,  usually  a  very  faint  shadowy 
image  of  the  lower  surface  will  be  observed,  slightly  out  of  sym- 
metry with  the  upper  surface.  Swinging  the  mirror  to  one  side 
or  decentering  the  iris  diaphragm  of  the  condenser  when  this  is 
possible,  and  noting  at  the  same  time  any  change  produced  in 
the  image,  will  show  that  the  image  of  the  upper  surface  has  the 
appearance  of  sliding  over  the  lower,  providing  the  objective  has 
sufficient  penetrating  power.  Under  these  conditions  the  trained 
observer  is  able  to  form  a  fairly  accurate  conjecture  as  to  the 
morphology  of  the  object  under  observation. 

Cleavage  planes,  infinitely  narrow  fissures  or  structures,  the 
arrangement  of  whose  elements  is  so  fine  and  delicate  as  to  be 
practically  indistinguishable  by  axial  light,  may  become  easily 
discernible  by  oblique  illumination;    but  as   intimated  above, 


2  Axial  Light 

a. 
O 


Fig.  8. 


32 


ELEMENTARY   CHEMICAL   MICROSCOPY 


the  character  of  the  information  thus  gained  is  necessarily 
closely  associated  with  the  resolving  power,  penetration  and, 
to  a  certain  extent,  the  size  of  field  of  the  optical  combination 
above  the  stage. 

DEVICES   FOR   ILLUMINATION  BY   TRANSMITTED   LIGHT. 

Condensers.  —  In  order  that  sufficient  light  may  enter  a  high 
power  objective  to  produce  an  image  of  such  a  degree  of  bright- 
ness as  to  be  easily  studied,  it  is  essential  that  some  device  or 

apparatus  shall  collect,  concen- 
trate and  send  through  the 
object  light  rays  at  an  angle 
which  will  fill  the  aperture  of 
the  objective. 

The  usual  construction  of 
this  device  is  shown  in  diagram 
in  Fig.  9  and  is  known  as  the 
A  bbe  condenser.  Condensers 
of  this  construction  with  two 
lenses  have  usually  a  numerical 
aperture,  when  employed  to 
their  full  extent,  of  1.20  and 
may  be  used  with  all  ordinary 
dry  objectives  and  with  oil  im- 
mersion objectives.  They  are 
designed  to  be  used  with  the 
plane  mirror.  In  the  case  of 
objectives  of  more  than  1.20 
N.A.,  a  three  or  more  lens  com- 
bination condenser  giving  1.40 
N.A.  should  be  chosen.  Con- 
densers used  to  their  full  aperture  usually  so  flood  the  field 
with  light,  in  the  case  of  dry  objectives,  as  to  necessitate  lower- 
ing them  or  closing  their  iris  diaphragms  or  both  until  only 
just  sufficient  light  rays  are  intercepted  by  the  objective  to  fill 
its  back  lens  and  thus  render  the  fine  details  of  the  illuminated 
object  most  distinct. 


Fig. 


Diagram  of  Abbe  Condenser; 
Axial  Light. 


ILLUMINATION   OF   OBJECTS;   ILLUMINATING   DEVICES       33 


In  the  diagram,  Fig.  9,  the  passage  of  the  light  rays  is  roughly 
indicated  for  a  position  of  the  Abbe  condenser  when  used  with 
an  objective  of  low  numerical  aperture.  The  iris  diaphragm 
is  shown  well  closed.  Usually  it  is  advisable  to  also  lower  the 
condenser.  Failure  to  employ  the  Abbe  condenser  in  the  proper 
manner  or  to  appreciate  the  fact  that  a  different  adjustment  is 
required  to  meet  different  prob- 
lems, is  doubtless  responsible  for 
more  errors  in  interpretation  in 
microscopic  examinations  than 
any  cause  other  than  exces- 
sive magnification.  Since  very 
few  dry  achromatic  objectives 
have  a  high  numerical  aperture 
it  is  evident  that  in  order  to 
obtain  the  best  results  it  will  be 
essential  with  all  such  optical 
combinations  to  close  the  iris 
diaphragm  of  the  Abbe  con- 
denser until  the  numerical  aper- 
ture is  no  greater  than  that  of 
the  objective.  It  will  be  found 
to  be  a  safe  general  rule  to  lower 

the  Abbe  condenser  and  to  close    Fig.  10.     Diagram  of  Abbe  Condenser; 

its  iris  diaphragm  to  a  diameter  0blique  LighL 

about  two-thirds  or  one-half  that  of  the  rear  lens  opening  of 
the  objective.  The  size  of  the  diaphragm  opening  may  easily 
be  adjusted  by  removing  the  ocular,  looking  into  the  tube  of 
the  microscope  and  closing  the  diaphragm  until  the  bright  disk 
of  light  is  reduced  one-half  or  two-thirds. 

Oblique  illumination  with  the  Abbe  condenser  is  quickest 
and  most  easily  obtained  by  the  method  suggested  by  Wright 
of  holding  a  finger  below  and  half  across  the  opening  of  the  con- 
denser; the  light  rays  then  take  the  path  roughly  indicated  in 
Fig.  10.  Or  we  may  drop  upon  the  swing-out  ring  attached 
to  the  bottom  of  the  condenser  mounting  a  half-disk  of  black 
paper  or  cardboard,  or  a  disk  provided  with  a  circular  opening 


34  ELEMENTARY   CHEMICAL   MICROSCOPY 

to  one  side  of  the  center.  The  disks  furnished  with  the  conden- 
ser, consisting  of  a  central  stop  with  narrow  slots,  yield  very 
oblique  illumination  but  a  black  background,  and  serve  an  en- 
tirely different  purpose  which  is  discussed  elsewhere  under  the 
head  Dark-ground  Illumination.  In  the  highest  grades  of  mi- 
croscopes the  substage  mounting  is  arranged  so  as  to  provide  a 
lateral  movement  of  the  iris  diaphragm  by  means  of  rack  and 
pinion.  Oblique  illumination  is  then  obtained  by  closing  the 
diaphragm  to  a  small  opening  and  racking  it  to  one  side. 

Oblique  illumination  is  often  essential  to  a  proper  interpre- 
tation of  structure  and  to  a  sharp  differentiation  of  refractive 
indices. 

The  ordinary  Abbe  condenser  is  corrected  for  neither  chromatic 
nor  for  spherical  aberration  and  although  it  answers  all  the 
purposes  of  illumination  in  ordinary  microscopy  with  standard 
objectives,  in  photomicrography  or  in  combination  with  objec- 
tives of  the  highest  grade  and  in  work  of  the  finest  kind,  its  use  is 
injudicious.  Recourse  should  be  had  in  such  cases  to  achromatic 
or  specially  constructed  condensers.  Since  investigations  of 
this  kind  are  rare  in  chemical  laboratories,  space  forbids  their 
consideration. 

In  accurate  crystallographic  studies  the  microscope  condenser 
must  be  especially  free  from  both  chromatic  and  spherical  aber- 
ration; and  instruments  for  this  class  of  work  are  never  provided 
with  condensers  of  the  Abbe  type,  but  are  always  fitted  with 
light-concentrating  devices  of  special  construction. 

It  is  essential  that  the  optic  axis  of  the  condenser  shall  coincide 
with  the  optic  axis  of  the  microscope,  or,  in  other  words,  the  con- 
denser must  be  accurately  centered.  In  the  low-priced  micro- 
scopes no  provision  is  made  for  any  adjustment  of  the  mounting, 
the  proper  position  being  fixed  by  the  manufacturer.  Not 
infrequently  through  carelessness  of  workmen  and  inadequate 
inspection  of  the  finished  instrument,  microscopes  are  sold 
whose  substage  condensers  are  so  badly  out  of  center  as  to  render 
them  unfit  for  high  grade  work. 

To  test  the  adjustment  of  an  Abbe  condenser  in  a  fixed  mount- 
ing, close  its  iris  diaphragm  to  the  smallest  obtainable  opening, 


ILLUMINATION   OF   OBJECTS;   ILLUMINATING   DEVICES       35 

raise  the  substage  as  far  as  it  will  go ;  insert  a  cross-hair  eyepiece 
in  the  body  tube  and  focus  with  a  very  low  power  upon  the 
diaphragm  opening.  The  diaphragm  opening  should  fall  at  the 
center  of  the  field  of  view  directly  under  the  cross-hairs,  con- 
centric with  their  point  of  intersection.  If  the  image  of  the 
opening  is  not  centrally  located  there  is  something  faulty  in  the 
construction  of  the  condenser  or  in  its  attachment  to  the  sub- 
stage,  or  in  the  alignment  of  objective  and  ocular. 

If  the  condenser  has  been  found  centered,  we  may  change  to  a 
high  power  objective  and  be  reasonably  sure  that  the  condenser 
will  be  centered  with  respect  to  the  objective,  providing  a  re- 
volving nose-piece  is  not  in  use;  but  if  the  objective  is  attached 
to  an  ordinary  nose-piece,  turning  from  one  objective  to  another 
usually  necessitates  a  readjustment  of  the  condenser.  With  high 
powers,  centering,  as  described  above,  is  impossible  and  it  will 
be  found  simpler  to  remove  the  ocular  and  hold  a  tripod  or 
pocket  magnifier  over  the  tube;  the  image  of  the  diaphragm 
opening  is  then  easily  seen  and  its  relative  position  ascertained. 

In  testing  for  proper  centering  it  is  important  that  the  mirror 
be  so  placed  as  to  yield  exactly  axial  light.  This  may  be  assured 
by  swinging  the  condenser  to  one  side  and  placing  upon  the 
stage  a  preparation  consisting  of  thin  gum  beaten  up  until  full 
of  air  bubbles;  a  very  tiny  air  bubble  is  selected  and  brought 
to  the  center  of  the  field,  it  appears  as  a  bright  spot  surrounded 
by  a  black  ring;  the  bubble  is  sharply  focused  and  the  mirror 
adjusted  by  proper  tipping  until  the  bright  spot  appears  exactly 
at  the  center  of  the  circular  black  ring.  The  light  is  now  exactly 
axial.  This  method  of  assuring  absolutely  axial  light !  is  the 
simplest  and  surest  available. 

Without  touching  the  preparation  or  the  mirror,  carefully 
swing  the  condenser  back  in  place,  raise  it  about  halfway  and 
slowly  raise  and  lower  the  body  tube  by  means  of  the  coarse 
adjustment,  closely  observing  at  the  same  time  the  appearance 
of  the  bubble  image.  If  the  light  still  remains  axial  with  the 
condenser  in  place  there  will  be  no  appreciable  swaying  of  the 
image  and  no  change  of  position  of  the  bright  spot  of  light.     If 

1  Gage,  The  Microscope,  p.  48,  10th  Ed.,  Ithaca,  1908. 


36  ELEMENTARY   CHEMICAL   MICROSCOPY 

the  image  sways  and  the  bright  spot  of  light  is  displaced  to  one 
side  of  the  center  the  Abbe  condenser  is  faulty  and  the  character 
and  the  amount  of  the  fault  will  be  indicated  by  the  magnitude 
of  image  displacement. 

In  the  better  grades  of  Abbe  condensers  the  mounting  is 
fitted  with  centering  screws,  which  permit  moving  the  combina- 
tion of  lenses  so  that  the  optic  axis  of  the  condenser  lens  becomes 
coincident  with  the  optic  axis  of  objective  and  ocular. 

The  simplest  method  for  easily  centering  adjustable  Abbe 
condensers  is  to  have  a  cap  made,  fitting  exactly  over  the  top 
lens  of  the  condenser;  at  the  exact  center  of  this  cap  an  exceed- 
ingly tiny  hole  is  drilled  falling  in  the  optic  axis  of  the  apparatus. 
The  microscope  is  focused  upon  this  hole,  illuminated  by  the 
light  transmitted  by  the  condenser  and  the  bright  spot  seen  is 
brought  by  means  of  the  centering  screws  so  that  its  center  is 
coincident  with  the  center  of  the  field. 

It  is  the  rule  to  always  use  the  plane  mirror  with  the  Abbe 
•condenser;  but  when  the  windows  of  a  laboratory  have  small 
panes  or  wide  cross  bars  it  is  often  impossible  to  properly  illu- 
minate an  object  with  the  plane  mirror  and  Abbe  condenser 
without  projecting  an  image  of  the  window  bars  into  the  field. 
Either  the  microscope  must  be  moved  very  close  to  the  window 
or  the  concave  mirror  must  be  used;  the  latter  plan  necessi- 
tates closing  the  iris  diaphragm  two-thirds  or  more  and  lowering 
the  condenser.  In  aggravated  cases  a  disk  of  ground  glass  may 
be  placed  below  the  condenser  or  in  front  of  the  mirror.  The 
use  of  a  disk  of  thin,  fine  ground  glass  will  in  fact  be  found  a 
distinct  gain  in  ordinary  practice  in  the  illumination  of  most 
objects.  By  its  use  softer,  clearer  and  more  easily  interpreted 
images  will  often  be  obtained  and  the  true  colors  of  objects 
will  be  more  easily  recognized. 

When  the  recognition  of  colors  becomes  important,  as,  for 
example,  in  microchemical  analysis,  the  student  must  remember 
that  the  image  obtained  in  the  microscope  by  illumination, 
by  mirror  and  Abbe  condenser  with  light  from  the  sky  will 
almost  never  show  the  true  colors  present  in  the  object.  To 
obtain  a  properly  colored  image,  slide  a  piece  of  pure  white 


ILLUMINATION  OF  OBJECTS;   ILLUMINATING    DEVICES       37 

card  at  an  angle  of  about  45  degrees  between  condenser  and 
mirror,  or  place  a  disk  of  ground  glass  in  the  ring  attached  to 
the  lower  part  of  the  condenser,  thus  obtaining,  in  part,  reflected 
light  and  a  gray  or  white  background. 

The  ring  attached  to  the  lower  part  of  the  condenser  and 
arranged  to  swing  aside  serves  to  carry  disks  of  blue  glass  to  be 
employed  when  working  with  artificial  light.  By  this  means  a 
much  less  fatiguing  illumination  is  obtained,  and  providing  the 
proper  intensity  of  cobalt  glass  is  at  hand,  white  light  giving 
proper  color  values  is  secured.  Blue  glass  should  always  be 
placed  below  the  condenser  when  working  with  yellow  artificial 
lights.  Most  manufacturers  supply  blue  glass  disks  with  all 
their  Abbe  condensers.  When  the  apparatus  is  to  be  employed 
in  photography,  yellow-green  glass  disks  are  furnished  to  be  used 
as  ray  filters. 

c — d.  Reflected  Light,  Axial  or  Oblique,  must  be  employed  for 
the  study  of  the  surfaces  of  opaque  objects  or  for  the  purpose 
of  ascertaining  the  surface  configuration  of  objects  of  any  nature. 

In  investigations  of  this  sort  the  preparation  may  be  illu- 
minated either  by  rays  of  light  whose  paths  are  oblique  to  the 
surface  of  the  object  and  also  to  the  optic  axis  of  the  microscope 
or  by  rays  whose  paths  are  parallel  (or  approximately  so)  to 
the  optic  axis  and  normal  to  the  surface  of  the  preparation. 

Oblique  light  rays  are  obtained  either  by  means  of  small 
reflectors  attached  to  the  objective  or  by  directing  upon  the 
object  the  rays  from  a  radiant  lying  above  the  plane  of  the  surface 
of  the  object.  When  a  radiant  is  employed,  as,  for  example,  an 
arc  lamp  or  a  Nernst  lamp,  a  condensing  lens  is  usually  inter- 
posed between  light  and  object  in  order  to  concentrate  the  light 
rays  and  facilitate  the  proper  placing  of  the  illuminating  beam. 
Illumination  by  a  reflecting  mirror  may  be  obtained  either  by 
means  of  the  mirror  of  the  microscope,  provided  its  swinging 
arm  is  long  enough  to  allow  raising  the  mirror  above  the  plane 
of  the  stage,  or  by  attaching  to  the  objective  a  silvered  metal 
paraboloid.  The  paraboloid  illuminator  was  very  popular  at 
one  time  but  has  been  almost  entirely  superseded  by  devices 
known  as  vertical  illuminators  (see  page  76)  in  which  the  re- 


38  ELEMENTARY  CHEMICAL  MICROSCOPY 

fleeting  surface  is  mounted  in  a  cell  attached  to  the  microscope 
just  above  the  objective.  In  these  devices  the  reflector  sends 
the  illuminating  beam  of  light  through  the  objective  which  acts 
as  the  condenser,  concentrating  the  light  rays  into  a  bright  spot 
of  light  upon  the  surface  of  the  object  at  a  point  lying  approxi- 
mately in  the  optic  axis  of  the  microscope.  From  the  surface 
of  the  object  the  rays  are  reflected  back  through  the  objective 
and  form  the  image  of  the  object  in  the  usual  manner. 

When  only  very  low  powers  are  required  for  the  examination 
of  a  specimen,  simply  holding  it  slightly  inclined  upon  the  stage 
will  send  sufficient  light  into  the  instrument  to  permit  a  thor- 
oughly satisfactory  study  of  the  coarse  details.  Slight  focusing 
up  and  down  will  answer  all  purposes. 

Since  reflected  axial  and  oblique  light  must  very  frequently 
be  employed  by  the  chemist  it  is  essential  that  he  should  thor- 
oughly understand  the  phenomena  exhibited  by  different  sur- 
faces illuminated  in  different  ways. 

If  we  are  dealing  with  a  highly  polished  mirror  surface  S, 
Fig.  ii  (as,  for  example,  a  polished  but  unetched  metallurgical 

specimen),  lying  in  a  plane  normal  to  the 
optic  axis  of  the  microscope,  and  we  il- 
luminate it  by  reflected  light,  it  is  obvious 
that  none  of  the  oblique  rays  ab,  cd  and 
ef  can  enter  the  objective  to  form  an  image 
since  the  angle  of  reflection  is  equal  to  the 
Fig.  ii.  Path  of  Oblique  angle  of  incidence.  The  surface  will  there- 
Light  Rays  striking  a  fore  appear  <far&.    The  more  nearly  a  per- 

Plane  Polished  Surface.    .  „       .  ,  ,  r 

feet  reflecting  surface  the  object  possesses, 
the  darker  it  will  appear.  It  will  remain  dark  until  the  ray  ef 
becomes  almost  parallel  to  the  optic  axis  and  therefore  prac- 
tically normal  to  the  surface  of  S.  Reflected  light  rays  now 
can  enter  the  objective  and  the  surface  appears  bright  and  shining. 
But  if  the  surface  of  the  object  illuminated  by  the  oblique 
rays  is  irregular  or  etched,  as  diagramed  in  Fig.  12,  then  the 
irregularities  will  appear  bright,  the  plane  or  polished  surfaces 
dark.  If  a  light  ray  a  strikes  a  series  of  tiny  minute  points  as 
at  D,  the  light  will  be  diffracted;  diffraction  patterns  will  be 


ILLUMINATION    OF   OBJECTS;    ILLUMINATING   DEVICES       39 

formed  in  the  field  of  the  microscope  and  the  true  structure  of 
the  object  at  this  point  will  prove  very  difficult  of  interpretation. 
When,  however,  axial  reflected  light  is  used,  that  is,  when  the 
illuminating  beam  strikes  the  polished  preparation  normal  to 
its  surface,  the  plane  surfaces  will  appear  bright,  the  irregularities 


Fig.  12.     Path  of  Oblique  Light  Rays       Fig.   13.      Path   of   Axial   Light   Rays 
striking  an  Irregular  Surface.  striking  an  Irregular  Surface. 


more  or  less  dark,  and  minute  projecting  irregular  points  will 
yield  diffraction  patterns;  for  as  shown  in  Fig.  13,  the  light 
rays  b  and  c,  striking  reflecting  surfaces,  are  turned  aside  at  such 
an  angle  as  to  preclude  their  entering  the  objective. 

Careful  consideration  of  the  above  described  phenomena  is 
absolutely  essential  to  a  correct  interpretation  of  the  structure 
of  the  material  being  studied.  To  determine  when  one  is  dealing 
with  depressions  and  when  with  elevations  when  working  with 
moderately  high  powers  and  vertical  or  oblique  illumination  is 
often  a  difficult  problem  which  is  further  complicated  for  the 
beginner  by  the  fact  that  the  image  seen  is  that  of  the  object  in 
a  completely  reversed  position. 

It  is  obvious  that  the  oblique  illumination  of  opaque  objects 
is  restricted  to  low  powers,  since  the  free  working  distance  of 
high  power  objectives  is  so  small  that  the  path  of  any  pencil  of 
light  which  will  strike  the  preparation  at  a  point  lying  in  the  line 
of  the  optic  axis  of  the  microscope  must  then  be  so  oblique  as 
to  be  approximately  parallel  to  the  surface  of  the  preparation. 

Light  rays  reflected  from  the  surfaces  of  anisotropic  crystals 
are  polarized,  but  are  not  noticeably  polarized  if  from  isotropic 
crystals.  It  therefore  often  proves  of  great  value  in  qualitative 
analysis  to  employ  polarized  light  for  the  illumination  of  objects 
to  be  studied  by  means  of  vertical  illuminators. 


40  ELEMENTARY  CHEMICAL  MICROSCOPY 

e.  Dark-ground  Illumination  is  usually  obtained  by  sending 
oblique  light  rays  into  the  preparation  from  below,  at  such  an 
angle  that  no  rays  directly  enter  the  objective.  This  is  accom- 
plished by  introducing  a  metal  stop  below  the  Abbe  condenser  so 
as  to  shut  out  all  central  rays  and  allow  only  rays  near  the  cir- 
cumference of  the  condensing  lenses  to  enter  the  preparation, 
or,  better,  by  substituting  for  the  Abbe  condenser  a  device  which 
will  reflect  rays  from  a  curved  surface  in  such  a  manner  as  to 
bring  them  approximately  to  a  focus.  In  preparations  thus 
illuminated  objects  appear  bright  upon  a  black  background. 

This  method  is  invaluable  for  demonstrating  the  presence  of 
very  minute  bodies  or  those  whose  index  of  refraction  is  so  very 
nearly  the  same  as  that  of  the  medium  in  which  they  occur  as 
to  cause  them  to  escape  detection  when  illuminated  by  trans- 
mitted light. 

It  is  generally  the  case  that  particles  of  a  diameter  of  one 
micron  or  less  require  dark-ground  illumination  for  their  demon- 
stration. 

If  the  obliquity  of  the  rays  from  the  illuminating  device  is 
very  great,  the  dark-ground  illuminator  becomes  an  "ultra- 
condenser"  and  may  be  employed  for  demonstrating  the  presence 
of  particles  less  than  0.2  p.  in  size. 

Dark-ground  illumination  is  employed  in  practice  in  the  ex- 
amination of  blood  for  the  presence  of  parasitic  organisms,  in 
the  study  of  bacteria,  in  the  biological  examination  of  water, 
in  the  study  of  foods,  fibers,  crystallization  phenomena,  tiny 
crystals,  submicroscopic  particles,  colloids,  etc. 

If  the  Abbe  condenser  is  to  be  employed  for  dark-ground 
illumination,  insert  one  of  the  dark  ground  stops  in  the  ring 
attached  to  the  bottom  of  the  condenser  mounting,  open  the 
iris  diaphragm  to  its  full  capacity,  and  screw  up  the  condenser 
in  its  mounting  until,  when  turned  in  place  and  the  substage 
is  racked  up  to  its  highest  point,  the  upper  lens  will  just  touch  a 
slide  laid  upon  the  stage.  A  drop  of  water  is  then  placed  be- 
tween the  condenser  lens  and  the  preparation  to  be  examined. 
It  is  always  essential  to  ascertain  the  thickness  of  object  slides 
which  yield  the  best  results  and  keep  this  value  for  future  refer- 


ILLUMINATION   OF   OBJECTS;   ILLUMINATING   DEVICES       41 

ence.  Special  dark-ground  illuminators  are  marked  by  the 
manufacturers  with  the  thickness  of  object  slide  for  which  they 
are  designed. 

The  use  of  an  Abbe  condenser  with  dark-ground  stop  as  a 
substitute  for  special  dark-ground  illuminators  is  not  to  be 
recommended  since  the  obliquity  of  the  rays  is  seldom  sufficient 
to  prevent  some  light  from  entering  the  objective.  The  results 
usually  obtained  are  likely  to  be  poor  and  unsatisfactory. 

Dark-ground  Illuminators  are  condensers  of  such  construction 
that  very  oblique  light  rays  are  caused  to  converge,  usually  by 
reflection,  and  to  so  strike  the  lower  surface  of  a  cover  glass 
placed  over  the  preparation  to  be  studied  as  to  be  totally  re- 
flected. To  prevent  axial  light  from  passing  through  the  illu- 
minator an  opaque  stop  is  placed  in  the  optic  axis  of  the  device. 

Cover  Glass 


Fig.  14.     Path  of  Rays  in  Reflecting  Condenser  for  Dark-ground  Illumination. 

The  field  is  therefore  black  or  nearly  so,  save  for  a  slight  halo  at 
its  edges,  while  the  objects  appear  bright  or  brilliantly  colored 
upon  a  dark  background. 

In  Fig.  14  a  simple  paraboloid  reflecting  illuminator  is  shown 
diagrammatically  in  section,  with  the  directions  of  the  light 
rays  so  exaggerated  as  to  make  clearer  the  reason  the  field  of 
view  is  dark. 


42 


ELEMENTARY   CHEMICAL   MICROSCOPY 


Sections  of  typical  illuminators  are  shown  in  Fig.  15,  A,  B, 
C,  D.  It  will  be  seen  that  although  the  construction  may  be 
different  in  different  types,  the  rays  emerge  at  approximately 
similar  angles.  In  illuminators  of  these  types  (B,  C,  D)  the 
curvatures  of  the  reflecting  surfaces  are  ground  after  mathe- 
matically calculated  curves  which  will  bring  the  light  rays  ap- 
proximately to  a  focus  at  a  point  upon  the  cover  glass.     In  the 


EST 


^g£s 


s: 


^^ 


Fig.  15.     Types  of  Dark-ground  Illuminators. 
A.     Nachet  et  Fils.  B.     Reichert. 

C.     Bausch  &  Lomb.  D.     E.  Leitz. 


diagrams  for  simplicity,  cover  glasses  and  preparations  have 
been  omitted.  The  cheaper  forms  of  dark-ground  illuminators 
fail  to  bring  the  rays  to  a  true  focus  and  instead  of  a  point  of 
light  upon  the  cover  glass  we  obtain  a  disk,  as  shown  in  an  exag- 
gerated manner  in  Fig.  14. 

An  exception  to  the  above  statement,  relative  to  the  construc- 
tion of  reflecting  condensers,  is  found  in  the  Beck1  dark-ground 

1  Made  by  R.  &  J.  Beck,  London. 


ILLUMINATION   OF   OBJECTS;    ILLUMINATING   DEVICES       43 


illuminator  in  which,  Fig.  16,  a  lens  is  combined  with  a  paraboloid 
to  bring  the  rays  to  a  proper  focus. 

The  Beck  illuminator  is  unique  in  that  it  permits  adjustment 
for  different  thicknesses  of  object  slides,  an  impossibility  with 
other  forms  of  paraboloid  illuminators. 
This  adjustment  for  slide  thickness  is 
accomplished  by  changing  the  distance 
between  the  focusing  lens  L  and  the  pa- 
raboloid P.  As  seen  in  the  diagram,  the 
illuminator  consists  of  two  parts,  the 
paraboloid  mounting  screwing  into  that 
which  holds  the  lens;  therefore  raising  or 
lowering  the  paraboloid  will  displace  the 
focal  point  /  and  bring  about  an  accom-  FlG- l6-  Beck  Adjustable 
modation  for  different  thicknesses  of  slides.  Dark-ground  illuminator. 

In  practice  it  is  rarely  possible  to  have  such  accurate  grinding 
that  all  the  rays  are  properly  deflected  and  none  enter  the  objec- 
tive. Only  those  rays  included  in  a  low  numerical  aperture  are 
available.  Hence  the  employment  of  an  objective  of  high  nu- 
merical aperture  and  very  short  working  distance  yields  a  field 
which  is  never  dark.  Since  practically  all  high  power  immersion 
objectives  are  made  with  as  high  numerical  apertures  as  possible, 
it  is  absolutely  essential  that  some  means  be  used  to  reduce 
their  numerical  aperture  below  1,  if  they  are  to  be  employed  in 
dark-ground  studies.  This  is  accomplished  by  introducing  into 
the  objective  mount  some  form  of  diaphragm:  or  specially  con- 
structed objectives  of  N.A.  less  than  1  may  be  purchased. 
Diaphragms  for  use  with  objectives  in  dark-ground  studies  are 
generally  supplied  by  the  manufacturers  of  reflecting  condensers 
for  introduction  into  the  special  objectives  to  be  used.  These 
funnel-like  diaphragms  are  not  interchangeable  and  can  be 
employed  only  for  the  objective  for  which  they  are  designed. 
Figs.  17,  18  and  19  show  three  different  types  and  forms  of  dia- 
phragms employed  for  this  purpose.  In  the  case  of  Fig.  17  the 
lens  mounting  is  unscrewed  just  back  of  the  back  lens  combina- 
tion and  the  funnel  diaphragm,  provided  with  male  thread,  is 
screwed  into  the  opening  tapped  into  the  upper  half  of  the  objec- 


44 


ELEMENTARY   CHEMICAL   MICROSCOPY 


tive  mounting.  In  the  case  of  Fig.  18,  the  objective  is  also 
unscrewed  just  above  the  back  lens  combination,  but  in  this 
case  the  diaphragm  is  merely  dropped  into  the  hole  in  the  lower 
half  of  the  mounting,  while  in  the  case  shown  in  Fig.  19,  the 
long  tubular  diaphragm  is  inserted  into  the  objective  from  above 


Fig.  17.  Fig.  18.  Fig.  19. 

Methods  of  Reducing  Numerical  Aperture  of  Objectives  for  Dark-ground  Studies. 

(D,  D,  D,  Removable  Diaphragms.) 

without  necessitating  any  separation  in  the  mounting  of  the 
objective  lenses.  By  means  of  these  diaphragms  the  numerical 
apertures  of  the  objectives  are  reduced  to  approximately  0.9 
or  0.95. 

In  order  to  obtain  the  maximum  resolving  power  with  dark- 
ground  illumination  Conrady  has  shown1  that  the  condenser 
must  have  not  less  than  three  times  the  numerical  aperture  of 
the  objective.  He  suggests  that  the  practical  resolving  power 
obtainable  may  be  expressed  as  equal  to  \  N.A.  objective  -f 
\  N.A.  condenser,  but  Reinberger  points  out  that  on  actual 
trial2  the  Conrady  formula  gives  results  about  25  per  cent  too 
low.  The  inexperienced  observer,  however,  will  find  that  the 
resolving  power  obtainable  in  his  work  will  conform  rather 
closely  with  the  Conrady  formula.  It  is  therefore  well  to  bear 
in  mind  that  in  dark-ground  illumination  studies  fine  details  of 
structure  are  to  be  discerned  only  with  the  greatest  difficulty 

1  Conrady,  J.  Quekett  Micro.  Club,  11  (1912),  475. 

2  Reinberger,  J.  Quekett  Micro.  Club,  11  (191 2),  503. 


ILLUMINATION   OF   OBJECTS;    ILLUMINATING    DEVICES       45 

and  will  require  extreme  care  in  adjusting  the  illumination  and 
in  selecting  the  proper  objectives.1 

It  is  evident  that  with  a  properly  selected  optical  combination, 
the  field  of  view  will  appear  black  or  very  dark,  while  any  objects 
present  will  appear  to  be  bright  and  self-luminous. 

The  more  oblique  the  rays  the  more  minute  the  particles 
may  be  whose  presence  will  be  revealed  by  their  diffraction 
patterns.  When  the  upper  limit  of  obliquity  is  reached  the 
illuminators  are  usually  designated  as  ultracondensers  and  the 
instruments  to  which  they  are  attached  are  then  known  as 
ultramicroscopes.  There  is  no  sharp  dividing  line  between 
ordinary  dark-ground  illumination  and  ultramicroscopic  illu- 
mination; the  one  gradually  merges  into  the  other.  In  all  ultra- 
microscopes  we  are  dealing  with  dark-ground  illumination,  but, 
on  the  other  hand,  few  dark-ground  illuminators  yield  light  rays 
sufficiently  oblique  to  demonstrate  particles  of  ultramicroscopic 


Fig.   20.      Types  of   Reflecting  Condensers   for  the   Study  of  Ultramicroscopic 

Particles. 


size.  Typical  ultracondensers  are  shown  in  Fig.  20.  A  com- 
parison of  the  indicated  light  ray  directions  in  these  with  those 
in  Fig.  15  will  disclose  that  their  inclination  is  considerably 
greater.  For  the  chemist  the  ultracondensers  are  of  far  more 
value  than  simple  dark-ground  illuminators  and  those  fitting  into 
the  substage  will  be  found  preferable  to  those  of  plate  form, 

1  Siedentopf  and  Zsigmondy  have  shown  (Ann.  d.  Phys.  [4]  10  (1903),  14)  that 
in  the  ultramicroscope  the  brilliancy  of  the  diffraction  disks  is  proportional  to  the 
product  of  the  squares  of  the  numerical  apertures  of  the  image-forming  and  illu- 
minating objectives. 


46 


ELEMENTARY    CHEMICAL   MICROSCOPY 


e.g.,  Fig.  21,  which  lie  upon  the  stage  of  the  microscope.  More- 
over, all  ultracondensers  can  be  employed  as  ordinary  dark- 
ground  illuminators,  the  only  drawback  in  routine  work  being 
that  they  require  more  careful  adjustment. 


£ 


^ 


:\ 


\_ 


p 


Fig.  21.    Simple  Dark-ground  Illuminator  for  Use  upon  the  Stage  of  the  Microscope. 

The  Adjustment  of  Dark-ground  Illuminators  for  use  requires 
close  attention,  chiefly,  to  four  conditions:  (i)  a  selection  of  a 
sufficiently  powerful  radiant  and  the  projection  of  a  spot  of 
light  large  enough  to  completely  fill  the  lower  opening  of  the 
illuminator;  (2)  the  employment  of  objectives  having  a  numer- 
ical aperture  never  greater  than  1.0;  (3)  the  use  of  object  slides 
of  the  thickness  for  which  the  illuminator  has  been  designed; 
(4)  accurate  centering  of  the  illuminator  with  respect  to  the 
optic  axis  of  the  microscope. 

An  examination  of  the  diagrams  (Figs.  15  and  20)  will  show 
that  theoretically  the  oblique  rays  meet  to  form  a  tiny  spot  of 
light  just  outside  the  apparatus  in  the  line  of  its  optic  axis.  It 
is  obvious  that  this  spot  should  lie  in  the  optic  axis  of  the  objec- 
tive and  the  ocular.  In  order  to  facilitate  centering,  a  tiny  circle 
is  usually  engraved  upon  the  upper  surface  of  the  glass  of  the 
illuminator;  this  circle  is  focused  with  a  low  power  and  is 
brought  to  the  center  of  the  field  of  the  microscope,  either  by 
means  of  centering  screws  c,  c,  Fig.  22,  provided  for  this  pur- 
pose, or  is  moved  by  the  fingers  when  a  stage  illuminator,  Fig. 
21,  is  placed  upon  the  stage.     If  the  microscope  is  provided  with 


ILLUMINATION   OF   OBJECTS;   ILLUMINATING   DEVICES       47 

a  revolving  nose-piece  the  objective  used  in  centering  should  be 
removed  and  the  high  power  to  be  employed  in  the  dark-ground 
studies  substituted  in  the  same  opening  in  order  that  there  shall 
be  no  change  in  the  relations  of  the  optic  axes.  When  employ- 
ing ultracondensers  of  the  highest  type  it  is  better  to  remove 
the  nose-piece  and  to  attach  to  the  body  tube  a  centering  adapter 
into  which  the  objective  is  screwed;  this  permits  accurate  cen- 
tering of  each  objective  used  and  therefore  much  better  optical 
conditions  are  obtainable. 


Fig.  22.     Paraboloid  Dark-ground  Illuminator  for  Use  below  the  Stage. 

In  order,  however,  that  the  objective  may  be  centered,  it  is 
essential  that  we  have  a  central  fixed  point  upon  the  stage  to 
which  we  may  refer.  Stands  to  be  employed  for  high-grade 
ultramicroscopic  work  should  be  provided  with  mechanical 
stages  with  graduated  coordinate  motion  and  a  centering  object 
slide,  carrying  at  its  center  a  tiny  cross.  When  placed  upon 
the  stage  so  that  the  different  scales  of  the  mechanical  stage 
occupy  the  positions  which  the  manufacturer  has  indicated  upon 
the  object  slide,  the  point  of  intersection  of  the  ruled  cross  will 
fall  exactly  in  the  axis  of  the  tube  of  the  microscope.  The 
objective  is  focused  sharply  upon  the  cross  and  if  the  center  of 
the  cross  does  not  fall  in  the  center  of  the  field  it  is  brought  there 
by  moving  the  screws  a,  a,  Fig.  24,  page  59. 

If  the  condenser  is  not  provided  with  an  engraved  circle  upon 
its  upper  surface  it  may  be  centered  by  placing  an  object  slide 
upon  the  stage  with  immersion  fluid,  usually  water,  between  it 
and  the  condenser;  the  light  spot  from  the  radiant  is  next  prop- 
erly adjusted  and  the  mirror  inclined  until  a  bright  spot  of  light 
appears  upon  the  object  slide.     The  condenser  is  raised  or  lowered 


48  ELEMENTARY  CHEMICAL  MICROSCOPY 

until  the  spot  of  light  attains  its  smallest  size.  Focus  upon  this 
tiny  spot  with  a  low  power  objective;  if  the  condenser  is  properly 
centered  the  spot  will  lie  at  the  center  of  the  field.  Should  it 
he  to  one  side,  bring  it  to  the  center  by  means  of  the  centering 
screws  or  center  the  objective  with  respect  to  the  point  of  light. 

Having  adjusted  the  condenser,  the  next  step,  if  the  device 
is  of  the  cardioid  type  (see  page  67),  is  to  ascertain  whether 
the  quartz  cell,  which  must  be  used  with  the  instrument,  is  in 
proper  condition  for  use.  Lay  the  quartz  cover  upon  the  cell 
and  press  it  down  very  carefully.  Notice  whether  there  appears 
at  the  zone  of  contact  between  cell  and  cover  a  series  of  colored 
concentric  rings.  If  the  pattern  does  not  consist  of  concentric 
circles,  but  appears  to  be  elliptical,  it  is  probable  that  the  cell  is 
not  level  with  respect  to  the  optic  axis.  Adjust  the  level  screws 
until  the  plane  of  the  cell  is  normal  to  the  optic  axis.  If  the 
eccentricity  of  the  rings  does  not  disappear,  the  trouble  lies  in 
the  objective  which  is  not  corrected  for  the  thickness  of  the  cover 
of  the  cell  being  used. 

A  powerful  source  of  light  is  essential.  Direct  sunlight  by 
means  of  a  clockwork  heliostat  is  ideal  but  seldom  available. 
The  next  choice  is  an  electric  arc  of  4  to  5  amperes  or  more,  for 
ordinary  dark-ground  examinations,  and  of  15  to  20  amperes  for 
ultramicroscopic  studies  of  colloids,  etc.  Useful  types  of  radi- 
ants will  be  found  described  on  page  132. 

The  more  powerful  the  radiant  the  smaller  the  particles 
which  can  be  demonstrated.  Siedentopf  estimates  that  direct 
sunlight  will  reveal  the  presence  of  particles  whose  diameters  are 
one-thirtieth  of  that  of  the  smallest  appreciable  with  the  ordinary 
arc  lamp. 

Since  the  light  rays  enter  these  reflecting  condensers  through 
an  annular  space,  there  being  an  opaque  stop  at  the  center,  it  is 
obvious  that  the  spot  of  light  reflected  from  the  mirror  of  the 
microscope  must  have  a  diameter  slightly  greater  than  this 
space,  otherwise  the  illuminator  will  not  properly  function;  for 
this  reason,  before  placing  the  illuminator  in  position  for  cen- 
tering, it  is  always  essential  to  examine  its  lower  surface  and 
ascertain  the  diameter  of  the  spot  of  light  necessary  to  completely 


ILLUMINATION   OF   OBJECTS;   ILLUMINATING   DEVICES       49 

fill  the  annular  entrance  space.  The  radiant  and  a  suitable 
condensing  lens  are  then  so  placed  as  to  yield  parallel  rays  and 
produce  a  spot  of  light  of  the  proper  size  and  intensity  at  the 
center  of  the  plane  mirror  of  the  microscope,  the  mirror  being 
so  inclined  as  to  reflect  the  light  rays  into  the  dark-ground  illu- 
minator. Dark-ground  illuminators  require  that  an  immersion 
fluid  be  placed  between  them  and  the  object  slide.  Usually 
freshly  filtered  water  is  sufficient,  although  homogeneous  im- 
mersion oil  sometimes  yields  better  results,  especially  with  illu- 
minators of  the  plate  type. 

In  applying  the  immersion  fluid  and  laying  the  object  slide 
in  place  great  care  must  be  taken  to  prevent  the  entrance  of 
air  bubbles  or  dust  particles. 

Because  the  light  rays  are  caused  to  emerge  from  the  illumina- 
tor at  such  an  angle  (determined  by  the  inclination  of  the  reflect- 
ing surfaces)  as  to  converge  to  an  axial  point  lying  just  above 
the  plane  of  the  object  upon  the  object  slide,  it  is,  of  course, 
essential  that  the  thickness  of  the  object  slide  be  known,  for  if 
too  thin  the  illuminating  rays  will  meet  too  far  above  the  material 
to  be  studied,  or  if  too  thick  the  focal  point  will  lie  too  low; 
for  these  reasons  optical  instrument  makers  mark  upon  the  de- 
vices the  object  slide  thickness  to  be  employed.     For  example: 


Bausch  and  Lomb  paraboloid  illuminator. 

Zeiss  paraboloid  condenser 

Reichert  reflecting  condenser 

Reichert  slip-in  reflecting  condenser 

Leitz  reflecting  condenser 

Zeiss  cardioid  condenser  for  quartz  cell.  .  . 


Thickness  of  object  slide. 


i  .40  to  i  .55  mm. 
1.0  to  1 .10  mm. 
0.7    to  1 .  10  mm. 

2.0  mm. 
less  than  1  mm. 

1 .2  mm. 


Absolutely  clean  object  slides  and  cover  glasses  are  essential 
and  great  care  must  be  exercised  in  wiping  off  the  immersion 
fluid  from  the  condenser  to  avoid  scratching  the  glass.  Lens 
paper  of  the  highest  grade  only  should  be  employed,  and  the 
wiping  off  of  the  fluid  should  be  done  with  the  least  pressure 
possible,  otherwise  fatty  material  from  the  fingers  may  be  forced 
through  the  pores  of  the  lens  paper  upon  the  glass.     A  mere 


50  ELEMENTARY   CHEMICAL   MICROSCOPY 

trace  of  grease  upon  the  glass  surface  will  lead  to  the  formation 
of  air  bubbles,  or  will  prevent  optical  contact  if  water  is  the  im- 
mersion fluid. 

The  preparation  to  be  studied  must  be  thin  and  must  be 
covered  with  exceptionally  clean  and  very  thin  cover  glasses. 
Covering  the  preparation  with  a  cover  glass  is  essential. 

In  order  to  expedite  the  adjustment  it  is  well  to  have  at  hand 
a  permanent  slide  of  some  material  which  yields  good  results 
with  dark-ground  illumination,  as,  for  example,  diatomaceous 
earth.  With  such  a  preparation  on  the  stage  the  radiant,  mi- 
croscope mirror  and  the  condenser  are  all  so  mutually  arranged 
as  to  yield  the  best  illumination  of  the  diatoms;  the  final  adjust- 
ment is  then  made  by  raising  or  lowering  the  condenser.  The 
test  slide  may  now  be  replaced  by  the  preparation  to  be  studied. 
Little  change,  if  any,  should  be  required  to  give  the  most  sat- 
isfactory results.  If  material  of  unknown  structure  or  com- 
position is  placed  upon  the  stage  without  a  prior  examination 
of  material  of  known  behavior  much  time  may  be  lost  in 
attempting  to  interpret  anomalous  appearances  due  to  improper 
illumination. 

Owing  to  the  exceedingly  complicated  diffraction  patterns  often 
obtained  with  dark-ground  illumination- great  difficulty  may  be 
experienced  in  arriving  at  a  correct  explanation  of  the  phenom- 
ena observed,  and  it  is  only  after  study  of  materials  of  known 
structure  that  it  is  safe  to  proceed  to  examinations  of  somewhat 
similar  material  of  unknown  structure. 

/.  Orthogonal  Illumination  is  a  term  applied  by  Zeiss  after 
Siedentopf  and  Zsigmondy  to  an  arrangement  of  radiant,  con- 
densing lenses  and  tiny  slit  such  that  the  light  rays  enter  the 
preparation  at  right  angles  to  the  optic  axis  of  the  microscope. 
The  presence  of  particles  is  thus  indicated  by  the  light  diffracted 
from  them,  the  particles  themselves  remaining  invisible  and  only 
the  diffraction  patterns,  which  may  be  relatively  large,  are  seen 
in  the  field  of  view.  This  mode  of  illumination,  as  well  as  that 
by  exceptionally  oblique  rays,  given  above  under  e,  applied  to 
microscopic  examinations  gives  us  instruments  commonly  called 
ultramicrosco  pes . 


ILLUMINATION   OF   OBJECTS;   ILLUMINATING    DEVICES        5 1 

Orthogonal  illumination  is  employed  in  the  study  of  colloids 
and  other  particles  in  suspension  in  liquids  and  for  the  study  of 
particles  in  transparent  or  translucent  solids,  such  as  glass,  etc. 
For  details  as  to  apparatus  and  their  use,  see  page  57. 

g.  Differential  Color  Illumination  by  the  method  of  Rhein- 
berger1  may  be  obtained  by  substituting  for  the  dark-ground 
wheel  stop  of  the  Abbe  condenser  colored  disks  of  transparent 
material,  using  a  darker  color  for  the  central  portion  and  sur- 
rounding this  disk  with  an  annular  ring  of  a  lighter  and  strongly 
contrasting  color.  The  object  will  then  appear  strongly  illu- 
minated, but  colored  upon  a  colored  background.  If,  for  exam- 
ple, the  central  disk  is  blue  and  the  -ring  red,  the  objects  will 
appear  red  upon  a  blue  background.  With  care  and  a  suitable 
choice  of  colors,  very  remarkable  results  may  be  obtained  which 
may  greatly  facilitate  the  study  of  certain  sorts  of  material. 

h.  By  Means  of  Ultraviolet  Light.  -  -  When  ultraviolet  rays 
impinge  upon  certain  substances  they  become  fluorescent  and 
glow  with  violet,  red,  green  or  bluish  light.  The  color  of  the 
fluorescence  is  peculiar  to  the  substance.  Since  comparatively 
few  bodies  exhibit  this  phenomenon  and  since  the  color  is  a 
further  aid  in  differentiation,  advantage  has  been  taken  of  this 
property  of  bodies  as  a  means  of  identification  of  such  sub- 
stances not  readily  recognized  when  present  in  low  per  cents  in 
mixtures.  To  permit  the  extension  of  this  method  to  minute 
amounts  of  material  the  "Fluorescence  Microscope"  has  been 
constructed.2 

Ordinary  glass  is  practically  opaque  to  ultraviolet  rays  but 
not  to  the  light  rays  resulting  from  the  fluorescing  of  the  sub- 
stance; the  ultraviolet  rays  however  readily  penetrate  quartz. 
We  have,  therefore,  only  to  substitute  quartz  for  glass  in  the 
condenser  in  order  to  concentrate  the  ultra  rays  on  the  object 
upon  the  stage.  It  follows  from  this  that  although  the  illu- 
minating devices  must  be  of  quartz,  as  also  the  object  slide  upon 
which  the  object  lies,  the  objective  and  ocular  may  be  those 
ordinarily  employed. 

1  J.  Roy.  Micro.  Soc,  1896,  373;  Spitta,  Microscopy,  London,  1909;  175-178. 

2  Made  by  C.  Reichcrt,  Vienna,  Austria. 


52 


ELEMENTARY   CHEMICAL   MICROSCOPY 


Either  a  carbon  arc  with  special  carbons  or  a  mercury  vapor 
lamp  may  be  employed  as  radiant. 

Fig.  23  shows  diagrammatically  the  construction  of  a  fluo- 
rescence microscope.  The  rays  from  the  radiant  R  are  concen- 
trated by  the  quartz  condensing  lens  Q,  then  pass  through  the 
Wood-Lehmann  filter  F  consisting  of  a  quartz  or  of  a  blue 
"Uviol"  glass  cell,  thence  the  rays  pass  to  the  reflecting  quartz 


.-fl- 


Fig.  23.     Reichert  Fluorescence  Microscope. 

prism  P  which  in  turn  reflects  them  into  the  quartz  lens  dark- 
ground  condenser.  This  device  brings  the  ultraviolet  rays  to  a 
focus  upon  the  object  supported  upon  the  stage  by  means  of  an 
object  slide  of  quartz  or  of  Uviol  glass.  Ordinary  glass,  besides 
being  practically  opaque  to  rays  of  very  short  wave  length,  as 
stated  above,  fluoresces  with  a  violet  or  bluish  tint  under  the 
action  of  the  ultraviolet  rays  and  cannot  therefore  be  employed 
as  a  support.  If  it  is  necessary  to  cover  the  preparation  ordi- 
nary glass  cover  glasses  may  be  employed,  but  it  is  unwise  to  do 
so  if  thin  quartz  cover  glasses  are  available. 

As  in  all  dark-ground  illuminators,  an  immersion  fluid  between 
condenser  and  object  slide  is  essential.  In  this  case  glycerine  is 
employed  (n  =  1.47). 

The  light  filter  whose  function  is  the  removal  of  waves  of 
long  wave  length,  affecting  the  eye  as  light,  consists  of  two  com- 
partments, one  filled  with  a  20  per  cent  copper  sulphate  solution, 
the  other  with  an  aqueous  solution  of  nitrosodimethyl  aniline 
(1:  12000). 

The  only  changes  in  construction  and  materials  lie  entirely 


ILLUMINATION   OF   OBJECTS;    ILLUMINATING   DEVICES        53 

in  the  illuminating  devices.  Any  microscope  permitting  the 
attachment  of  a  dark-ground  illuminator  whose  lenses  are  made 
of  quartz  may  be  converted  into  a  fluorescence  instrument. 

Although  this  system  of  illumination  is  still  so  new  as  to  have 
been  tried  by  but  very  few  workers,  its  future  development 
seems  assured  and  its  usefulness  in  qualitative  chemical  analysis 
of  minute  fragments  of  material  to  be  unquestioned.1 

It  is  valuable  not  only  in  the  analysis  of  inorganic  material, 
such  as  crushed  minerals,  soils,  mixtures  of  tiny  crystals,  etc.,  but 
is  of  equal  value  in  organic  analysis,  in  the  examination  of  foods 
for  adulteration  and  even  in  the  microscopy  of  drinking  water. 

1  See  Heimstadt,  Das  Fluoreszenz-Mikroskop,  Zeit.  f.  wiss.  Mikros.,  28  (191 1), 
330;  Wasicky,  Das  Fluoreszenz-Mikroskop  in  der  Pharmakognosie,  Pharm.  Post, 
(1913);  Lehmann,  H.,  Das  Lumineszenz-Mikroskop,  seine  Grundlagen  und  seine 
Anwendungen,  Zeit.  f.  wiss  Mikros.,  30  (1913)  41 7- 


CHAPTER  IV. 
ULTRAMICROSCOPES. 

APPARATUS  FOR  THE   STUDY   OF   ULTRAMICROSCOPIC 

PARTICLES. 

Ultramicroscopes.  —  Attention  has  already  been  called  to 
the  fact  that  the  compound  microscope  with  transmitted  axial 
light  will  resolve  tiny  particles  in  suspension  in  a  liquid  only 
when  there  is  a  certain  appreciable  difference  between  the  re- 
fractive index  of  the  particles  and  that  of  the  liquid,  and  when 
the  diameters  of  the  particles  are  greater  than  half  the  value  as- 
signed to  the  shortest  wave  lengths  producing  the  effect  of  light 
upon  the  normal  human  eye.  We  have  also  seen  that  if  instead 
of  axial  light,  oblique  rays  are  employed  the  ability  to  discern 
minute  particles  and  intricate  structure  is  greatly  increased, 
especially  if  the  obliquity  of  the  rays  is  such  as  to  yield  an  illu- 
minated object  upon  a  black  background.  If  the  degree  of 
inclination  of  the  illuminating  rays  be  still  further  increased 
and  the  source  of  the  rays  a  powerful  radiant  and  the  objective 
employed  one  of  low  numerical  aperture,  only  light  diffracted  by 
the  object  will  enter  the  objective;  the  phenomenon  known  as 
the  "Tyndall  effect"  results,  so  familiar  in  the  scintillating  dust 
particles  visible  when  a  ray  of  sunshine  enters  a  tiny  opening 
in  a  darkened  room  or  cell.  The  existence  of  these  infinitely 
minute  particles  in  suspension  in  the  air  is  manifest  to  the  naked 
eye  through  that  phenomenon,  although  even  a  high-power 
microscope  fails  to  resolve  them.  The  ultramicroscope  is  merely 
the  adaptation  of  this  Tyndall  effect  to  microscopic  illumination. 
As  a  result,  the  existence  may  be  demonstrated  of  particles 
almost  one  thousand  times  smaller  than  is  possible  by  means  of 
the  most  powerful  instrument  employed  in  the  usual  manner. 

It  is  obvious  that  under  the  illumination  of  these  very  oblique 
rays,  light  alone  which  has  been  diffracted  or  reflected  by  the 

54 


ULTRAMICROSCOPES 


55 


particles  enters  the  microscope  and  eventually  the  eye  of  the 
observer,  and  that  therefore  he  never  sees  the  particles  them- 
selves, but  merely  a  diffraction  disk  of  light.  We  know  of  the 
existence  of  these  particles  through  the  same  manifestation  of 
more  or  less  scintillating  points  of  light  that  we  see  in  the  fixed 
stars  on  a  moonless  night.  As  hereinbefore  stated  the  image 
of  a  point  of  light  is  a  diffraction  disk  surrounded  by  alternate 
dark  and  bright  rings.  These  diffraction  disks  appear  to  be 
in  rapid  motion.  They  appear  to  spin,  to  expand  or  contract 
and  are  endowed  with  a  constant  vibratory  movement.  This 
is  due  to  the  fact  that  exceedingly  minute  particles  suspended 
in  a  liquid  exhibit  a  constant  vibratory  and  rotatory  motion, 
long  called  the  Brownian  movement  and  now  known  to  be  as- 
sociated with  and  a  manifestation  of  what  we  commonly  term 
molecular  vibration  or  bombardment.  The  presence  of  disin- 
tegrating or  so-called  "digestive"  colloids  increases  the  Brownian 
motion,  while  electrolytes  by  reason  of  their  causing  agglutina- 
tion tend  to  decrease  the  amplitude  of  the  paths  of  vibration. 

In  the  few  years  that  ultramicroscopic  research  has  become 
possible  a  large  number  of  investigations  have  been  made  upon 
the  amplitude  of  the  paths  of  vibration  of  the  finest  of  these 
infinitely  small  suspended  particles,  with  the  result  that  the 
measurements  made  agree  very  closely  with  the  theoretical 
values  computed  for  the  amplitudes  of  vibration  of  the  molecules. 
Agencies  which  increase  molecular  vibration,  such  as  heat, 
dilution  and  consequent  reduction  of  viscosity,  increase  the 
Brownian  movement.  Hence,  we  find  under  the  ultramicroscope 
the  suspended  particles  in  a  gas  (as,  for  example,  in  smoke) 
in  much  more  rapid  motion  than  in  a  liquid,  while  in  a  solid  the 
Brownian  movement  is  visible  only  with  the  greatest  difficult) 

Since  the  tiny  particles  in  suspension  are  being  bombarded 
on  all  sides,  the  motion  imparted  to  them  must  be  the  resultant 
of  the  forces  acting;  we  therefore  find  them  spinning  rapidly  as 
well  as  moving  to  and  fro.  Some  authors  have  even  suggested 
that  the  term  kryptokinetic  motion  be  assigned  to  the  rotatory 
movement  to  distinguish  it  from  the  oscillating  Brownian  vi- 
bration. 


56  ELEMENTARY   CHEMICAL   MICROSCOPY 

The  amplitude  of  the  Brownian  movement  may  be  ascertained 
by  means  of  a  net  ruled  eyepiece  micrometer  calibrated  in  the 
usual  manner.  Space  forbids  a  discussion  of  the  experimental 
details. 

{Note.  The  student  should  read  the  excellent  summary  of 
the  then  known  facts  relative  to  the  Brownian  movement  given 
by  Rutherford  in  Science,  30  (1909),  289-302.) 

The  light  emanating  from  the  particles  is  polarized,  the  in- 
tensity of  polarization  increasing  with  the  decreasing  size  of 
the  particles.  This  fact  enables  us  to  differentiate  between 
light  diffracted  by  the  particles  and  light  emanating  from  flu- 
orescent bodies,  since  fluorescent  light  is  not  polarized.  A  well- 
equipped  ultramicroscope  must  therefore  include  a  device  for 
the  projecting  of  polarized  light  into  the  preparations  and  an 
analyzer  for  the  study  of  the  light  rays  forming  the  image  in  the 
microscope.  But  it  must  be  remembered  that  even  in  the  highest 
developed  types  of  the  ultramicroscope  tiny  particles  in  suspen- 
sion are  discernible  only  when  the  refractive  indices  of  these 
particles  are  different  from  that  of  the  medium  in  which  they 
are  suspended;  otherwise,  no  light  will  be  diffracted  from  them. 
Therefore,  although  a  medium  may  appear  to  be  "  optically 
empty"  when  viewed  in  the  ultramicroscope,  it  by  no  means 
follows  that  there  are  no  so-called  " colloids"  in  suspension. 
To  meet  this  difficulty  and  to  extend  the  range  of  the  ultrami- 
croscope, W.  Ostwald1  has  suggested  that  monochromatic  light 
be  employed.  This  suggestion  is  based  upon  the  fact  that 
although  two  substances  may  have  an  identical  value  for  their 
refractive  indices  for  white  light,  with  light  rays  of  certain 
definite  wave  length  the  indices  may  be  sufficiently  different 
to  permit  the  illuminating  rays  to  render  the  tiny  particles 
manifest. 

To  the  smallest  particles  visible  in  the  ultramicroscope  the 
terms  micellae,  ultramicrons  or  submicrons  are  sometimes  given. 
Particles  still  smaller  and  therefore  invisible  in  the  ultra-micro- 
scope are  called  amicrons. 

The  earliest  practical  instrument  may  be  said  to  be  the  Slit 

1  Ostwald,  W.,  Zeit.  f.  Ind.  Kol.,  11  (1912),  290. 


ULTRAMICROSCOPES  57 

Ultramicroscope  of  Siedentopf  and  Zsigmondy.  At  first  sight 
this  instrument  might  be  thought  to  be  also  the  most  efficient, 
in  that  the  path  of  the  illuminating  rays  entering  the  object  cell 
is  at  right  angles  to  the  optic  axis  of  the  observing  microscope; 
but  it  must  be  remembered  that  owing  to  internal  reflections 
and  the  impossibility  of  obtaining  a  perfectly  black  background 
the  field  is  never  sufficiently  black  to  render  very  feeble  diffrac- 
tion evident.  This  failure  to  obtain  a  black  background  is  due, 
as  first  stated,  to  internal  reflection  on  the  one  hand  and  upon 
the  other,  to  the  fact  that  the  beam  of  light  entering  the  cell  is 
usually  of  such  a  diameter  that  when  the  objective  is  focused 
upon  it  there  is  always  a  plane  below  that  in  focus  which  con- 
tains bright  particles.  Moreover,  this  trouble  is  aggravated  for 
the  reason  that  it  is  essential  to  use  objectives  of  long  working 
distance  and  great  penetrating  power.  These  difficulties  are 
largely  eliminated  in  the  more  recently  perfected  ultracondensers 
of  the  dark-ground  illuminator  types,  since  in  these  devices  not 
only  is  the  background  blacker  but  the  light  entering  the  liquid 
under  observation  is  greater  in  quantity.  For  example,  in  the 
cardioid  condenser,1  the  makers  estimate  that  its  light-concen- 
trating power  is  approximately  twenty  times  that  of  the  slit 
ultramicroscope. 

In  spite  of  this  advantage  of  the  ultracondenser  to  demon- 
strate the  presence  of  particles  in  suspension  greatly  beyond 
the  limit  of  instruments  of  the  slit  type,  preference  should  be 
given  to  the  latter  form  for  general  use  in  the  chemical  laboratory 
when  only  a  single  type  of  instrument  can  be  purchased,  because 
of  the  fact  that  the  slit  microscope  is  universal  in  its  application, 
serving  equally  well  for  solids,  liquids,  gases  or  vapors,  and  for 
hot  or  cold  preparations,  while  the  reflecting  condenser  types 
are  confined  to  the  study  of  thin  films  of  liquid  at  room  tempera- 
ture (or  in  certain  restricted  cases  to  the  study  of  tiny  transparent 
fibers)  .2 

The  Slit  Ultramicroscope  consists  of  an  ordinary  compound 
microscope,  a  special  cell  of  black  glass  with  small  windows  at 

1  Made  by  Carl  Zeiss,  Jena. 

2  Gaidukov,  Zeit.  angw.  Chem.,  21,  I  (1908),  393. 


58  ELEMENTARY    CHEMICAL   MICROSCOPY 

right  angles  to  one  another  and  an  illumination  device  for  pro- 
jecting a  tiny  beam  of  light  into  the  cell  in  a  line  at  right  angles 
to  the  optic  axis  of  the  microscope.  The  tiny  beam  of  light  is 
obtained  by  means  of  small  projection  lenses  and  an  adjustable 
slit.  To  distinguish  this  type  of  illumination  from  others  com- 
monly employed  in  microscopy,  the  term  "orthogonal  illumina- 
tion" has  been  proposed.  It  is  obvious  that  in  this  system  no 
direct  light  can  enter  the  objective  but  only  such  rays  as  are 
diffracted  by  the  particles  in  suspension  in  the  liquid  contained 
in  the  cell. 

The  form  and  arrangement  of  the  component  parts  of  the 
slit  ultramicroscope  naturally  differ  according  to  the  optical 
firm  manufacturing  the  instrument.  One  of  the  best  known 
and  most  frequently  used  types  is  that  shown  in  Fig.  24.1  This 
instrument  consists  of  an  optical  bench  B,  at  one  end  of  which 
is  placed  an  arc  lamp  R  and  at  the  other  a  compound  microscope. 
Between  the  lamp  and  the  microscope  there  are  a  series  of  con- 
densing lenses  and  an  adjustable  slit.  The  light  rays  emanating 
from  the  arc  are  collected  by  the  spherically  and  chromatically 
corrected  lens  Ci  of  80  millimeter  focus,  so  placed  as  to  project  a 
very  bright  image  of  the  crater  of  the  arc  upon  the  slit  S.  In 
ordinary  use  this  slit  has  its  length  in  a  horizontal  position,  the 
width  being  controlled  by  the  micrometer  screw  with  graduated 
head  G,  while  the  length  of  the  slit  is  regulated  by  the  screw  s. 
After  passing  through  the  slit  the  light  rays  enter  the  lens  C2, 
having  a  focal  length  of  55  millimeters,  whose  function  is  to 
project  a  reduced  image  of  the  slit  into  the  condenser-objective 
C3.  Since  both  slit  and  lens  C2  are  movable  forward  and  back 
upon  the  optical  bench,  the  lens  C2  serves  a  double  purpose, 
projection  and  adjustment  of  the  magnitude  of  the  light  beam 
entering  C3.  The  objective  C3  projects  into  the  preparation 
contained  in  the  cell  of  black  glass  U  a  tiny  conical  beam  of 
light  at  right  angles  to  the  optic  axis  of  the  microscope  M.  To 
prevent  any  side  light  from  entering  the  preparation,  lenses  Ci 
and  C2  are  small  and  are  mounted  in  blackened  metal  screens; 
as  a  further  precaution  a  large  metal  screen  D  with  tubular 

1  Manufactured  by  Carl  Zeiss,  Jena. 


ULTRAMICROSCOPKS 


59 


sixy  oijdQ 


a 
8 


1 


t 

CI 

6 


6o 


ELEMENTARY   CHEMICAL   MICROSCOPY 


opening  or  adjustable  diaphragm  is  introduced  between  the 
radiant  and  Ci-  The  objective  C3  screws  into  a  tube  fitting  into 
the  sleeve  T  and  may  be  slid  forward  and  back  for  coarse  ad- 
justment. A  very  sensitive  forward  and  back  movement  is 
further  provided  by  the  fine  adjustment  screw  Vi.  A  second 
fine  adjustment  to  the  right  and  left  for  accurately  centering 
the  illuminating  cone  of  light  is  obtained  by  the  screw  V2.  By 
means  of  these  two  screws  it  is  possible  to  adjust  the  tiny  beam 
entering  the  material  to  be  studied,  in  such  a  manner  as  to 
ensure  the  focal  point  of  the  condensing  objective  C3  falling  in 
the  line  of  the  optic  axis  of  the  observing  microscope  M,  and 
therefore  have  the  whole  of  the  tiny  beam  lying  across  the  exact 
center  of  the  field  of  view. 


Fig.  25.     Biltz  Cell. 

To  the  lower  end  of  the  body  tube  of  the  microscope  is  at- 
tached an  adapter  A  with  centering  screws  a,  a,  providing  a 
device  for  accurately  centering  the  objective  O  (see  page  47). 

The  liquid  containing  suspensoids  is  conveniently  placed  for 
examination  in  a  Biltz  cell,  Fig.  25,  or,  when  the  short  piece  of 


Fig.  26.     Biltz-Thamae  Cell. 


rubber  tubing  which  is  attached  to  the  end  of  the  tube  is  objec- 
tionable because  of  its  possible  action  on  the  colloids,  a  Biltz- 
Thomae  cell,  Fig.  26,  may  be  substituted.  In  both  of  these 
cells  the  essential  feature  is  the  central  dark  glass  chamber  of 


ULTRAMICROSCOPES 


6l 


about  3  millimeters  internal  diameter,  provided  with  two  small 
windows  at  right  angles  to  each  other  —  these  two  windows 
consist  of  either  thin  glass  or,  better,  of  very  thin  quartz  disks 
cemented  in  place.  The  passage  of  the  beam  of  light  through 
one  of  these  cells  is  shown  in  the  diagram,  Fig.  27.  No  light 
other  than  that  diffracted  from  the  particles  in  suspension  in  the 
liquid  can  enter  the  observing  microscope.  The  cell  is  usually 
attached  to  the  microscope  objective  by  a  special  cell  holder; 


C|?p*2^g 

-^ 

- 

A 

_ 

I 

?'\W 

•  j\-Y — =, — J  g 

— ' 

0\* 

— ""     !ZZ-4    / 

E^9§5? 

V k  -.'g^oa*. \# 

r— 1 

10 

Fig.  27.     Illuminating  Rays  in  the  Cell  of  the  Slit  Ultramicroscope. 


this,  however,  is  open  to  the  serious  defect  of  difficulty  in  focus- 
ing and  that  cells  purchased  at  different  times  are  not  exactly  of 
the  same  thickness  of  wall,  and  hence  the  center  of  the  upper 
window  will  not  fall  in  the  optic  axis  of  the  microscope.  For 
these  reasons  the  author  prefers  to  support  the  cells  upon  an 
elevating  mechanical  stage  P,  as  shown  in  Fig.  24.  This  arrange- 
ment permits  the  shifting  and  easy  adjustment  of  the  cell,  so  that 
its  upper  window  is  exactly  centered  with  respect  to  the  optic 
axis  of  the  observing  microscope.  The  cell  is  held  in  place  by 
the  spring  clips  c.  The  stage  supporting  the  cell  U  may  be 
raised  and  lowered  by  means  of  a  knurled  nut  q.  The  nut  p 
clamps  the  stage  in  place  while  the  screws  Wi  and  W2  serve 
to  move  P  forward  or  back  and  to  the  right  or  left. 

One  of  the  most  serious  defects  of  the  Biltz  cell  is  the  difficulty 
of  properly  cleaning  it  after  use,  especially  when  there  has  been 
deposition  of  a  colloidal  film  upon  the  windows.  Treatment 
with  a  proper  solvent  and  long  washing  is  imperative.  Before 
introducing  a  liquid  for  examination  it  is  always  best  to  pour  a 
little  alcohol  through  the  cell  and  to  follow  this  with  the  alco- 
holic solution  to  be  studied,  or  if  aqueous  suspensions  are  to  be 


62 


ELEMENTARY   CHEMICAL   MICROSCOPY 


Fig.  28. 


Simple  Cell  for  Use  with  Slit 
Ultramicroscopes. 


employed,  displace  the  alcohol  with  distilled  water  free  from  all 
fatty  or  greasy  matter  and  then  introduce  the  colloidal  solution. 
This  process  is  usually  essential  in  order  that  the  liquid  to  be 
examined  shall  come  into  perfect  contact  with  the  windows  of 
the  cell  with  no  interfering  film  and  no  air  bubbles. 

A  much  cheaper  and  simpler  cell  is  shown  in  Fig.  28.1  It 
consists  of  a  tube  of  black  glass  with  central  swelling  and  win- 
dows at  right  angles  to 
each  other.  These  win- 
dows are  either  of  glass 
or  of  quartz,  the  latter 
being  preferable,  since 
glass  is  slightly  fluores- 
cent. For  use,  two  pieces 
of  rubber  tube  are  at- 
tached as  shown  by  the 
dotted  lines.  These  little 
cells  give  excellent  results  with  gases  and  vapors  and  may  also  be 
employed  for  the  study  of  such  solutions  as  will  not  be  affected 
by  contact  with  rubber.  For  preliminary  examinations  they  are 
far  more  convenient  than  the  Biltz  cell  and  like  it  can  easily  be 
held  in  place  on  the  type  of  stage  shown  in  Fig.  24  by  thin  metal 
clamps  or  rubber  bands.  Moreover,  these  cells  are  more  easily 
cleaned  and  are  relatively  inexpensive. 

When  solids  are  to  be  examined,  as,  for  example,  specimens 
of  glass,  it  is  important  that  there  be  two  sides  of  the  preparation 
which  meet  at  as  nearly  right  angles  in  as  sharp  an  edge  as  is 
possible.  The  reason  for  this  will  readily  be  understood  by  re- 
fering  to  the  diagram,  Fig.  29.  If  the  sides  do  not  meet  in  a 
sharp  edge  as  shown  at  a,  but  form  an  obtuse  angle  or  rounded 
edge  b,  the  beam  of  light  must  be  lowered  below  b.  If  this 
is  done,  the  beam  of  light  R  will  lie  too  low  to  be  focused,  even 
if  the  lower  lens  of  the  objective  is  brought  into  actual  contact 
with  the  upper  surface  of  the  object.  In  this  case  the  beam  lies 
beyond  the  working  distance  of  the  objective.  Should  we  at- 
tempt to  bring  R  within  the  range  W,  as  indicated  in  the  lowest 

1  Made  by  E.  Leitz,  Wetzlar. 


ULTRAMICROSCOPKS 


63 


diagram,  diffraction,  refractions,  reflections  and  dispersions  take 
place  of  such  characters  and  to  such  degrees  as  to  render  the 
detection  of  micellae  impossible. 

No  suggestions  as  to  optical  combinations  or  size  and  intensity 
of  the  illuminating  light  beam  may  be  given  which  will  be  applic- 
able to  all  materials.  As  in  all 
other  cases  of  microscopic  investi- 
gation, the  proper  conditions  must 
be  experimentally  ascertained  for 
each  preparation  examined,  but  it 
is  a  safe  rule  to  always  avoid  too 
large  a  slit  and  too  high  a  magni- 
fication. 

For  the  slit  ultramicroscope  as 
made  by  Zeiss  two  objectives  are 
specially  constructed,  a  dry  7  mil- 
limeter, 0.4  N.A.  achromatic  ob- 
jective for  the  study  of  solids,  and 
a  4.4  millimeter  water  immersion 
of  0.75  N.A.,  for  use  with  cells 
containing  solutions.  A  good  gen- 
eral outfit  should  include  oculars, 
1,  6,  8,  12  and  18. 

When  polarized  light  is  necessary 
in  the  study  of  colloidal  reactions 
a  nicol  prism  as  polarizer  mounted 
upon  a  saddle  stand  is  placed  be- 
tween the  lens  Ci  and  the  slit  S. 
The  analyzer  is  then  placed  as 
usual  above  the  ocular  of  the  mi- 
croscope M. 

To  adjust  the  illuminating  beam  of  light  used  with  the  slit 
ultramicroscope  shown  in  the  diagram,  screw  the  condenser- 
objective  C3  into  its  holder  T.  Place  the  projection  lens  C2 
at  about  10  to  12  centimeters  from  the  end  of  T,  place  the 
adjustable  slit  approximately  12  centimeters  from  C2,  the  projec- 
tion lens  Ci  about  12  to  15  centimeters  from  the  slit,  the  dia- 


Fig.  29.  The  Necessity  of  having 
Two  Sides  at  Right  Angles  in  the 
Object  for  Ultramicroscopic  Study. 


64  ELEMENTARY   CHEMICAL   MICROSCOPY 

phragm  D  1 2  to  1 5  from  Ci  and  the  arc  lamp  so  that  its  carbons 
are  about  8  centimeters  from  D.  Turn  on  the  current  adjusting 
the  rheostat  so  as  to  employ  a  current  consumption  of  approxi- 
mately 10  amperes  and  see  that  the  +  carbon  is  the  horizontal 
one.  Later  when  in  the  prosecution  of  studies  raise  the  current 
to  one  of  15  or  even  20  amperes.  Move  the  lens  Ci  backwards 
and  forwards,  at  the  same  time  holding  a  piece  of  dull  black 
glass,  dull  black  paper,  or  a  piece  of  ground  glass  in  front  of  the 
slit,  until  a  position  is  obtained  which  projects  an  image  of  the 
arc  of  maximum  brightness  upon  the  black  screen  and  of  such  a 
size  as  to  completely  fill  the  slit  opening.  Set  the  slit  so  that 
the  micrometer  screw  G  is  up  as  shown  in  the  figure,  and  adjust 
the  opening  to  about  1  millimeter  by  1.5  millimeters,  its  length 
being  horizontal.  Now  hold  a  black  screen  against  the  end  of  T 
and  move  C2  back  and  forth  until  a  very  bright  and  sharp  image 
of  the  slit  is  obtained,  adjusting  d  again  slightly  if  necessary. 
Next  hold  the  black  screen  so  that  its  surface  lies  in  the  plane 
of  the  optic  axis  of  the  observing  microscope  and  adjust  the 
objective  C3  so  that  a  very  bright,  uniform  spot  of  light  a  little 
less  than  1  millimeter  in  diameter  is  obtained.  Turn  the  fine 
adjustment  V2  until  the  spot  of  light  falls  in  the  optic  axis  of  M. 
For  the  final  adjustment  of  the  apparatus  the  cell  may  be  filled 
with  a  liquid  which  contains  colloids  yielding  brilliant  diffraction 
patterns  or  with  a  slightly  alkaline  solution  of  fluorescein.  The 
path  of  the  illuminating  beam  is  thus  easily  seen.  Focus  upon  it, 
using  a  low  power  objective  and  No.  1  eyepiece  and  by  means  of 
Vi  and  V2  adjust  the  beam  so  that  it  passes  through  the  center 
of  the  field  as  a  narrow  thread  of  light  with  its  minimum  diameter 
at  the  center  of  the  field.  Replace  the  material  employed  for  ad- 
justing by  the  substance  to  be  studied.  The  only  adjustment 
which  should  now  be  required  will  be  the  diameter  of  the  slit; 
if  there  appears  to  be  required  a  marked  change  in  slit  diameter 
it  is  probable  that  following  this  change  there  may  be  required 
slight  changes  of  Vi  and  Y2. 

If  the  beginner  will  proceed  as  indicated  little  difficulty  will 
be  experienced  in  adjusting  the  slit  ultramicroscope  for  use. 
The  most  annoying  feature  is  the  change  in  the  position  of  the 


ULTRAMICROSCOPES  65 

crater  of  the  electric  arc,  and  consequently  unequal  illumina- 
tion of  the  slit  results  or  there  is  a  failure  (due  to  a  flickering  arc) 
of  the  spot  of  light  to  remain  centered  upon  the  slit.  Holding 
the  black  screen  against  the  lens  C2,  on  the  side  toward  the  slit, 
from  time  to  time,  will  show  when  the  arc  needs  adjusting, 
since  there  should  appear  a  spot  of  light  of  uniform  intensity 
and  in  the  proper  position  to  fall  concentric  with  the  optic  axes 
of  Ci,  C2,  C3. 

When  dealing  with  exceedingly  fine  colloidal  particles  it  is 
often  an  advantage  to  cut  off  the  lower  half  of  the  beam  by 
means  of  a  screen  mounted  upon  a  saddle  stand  and  placed 
between  S  and  C2,  the  upper  horizontal  edge  of  the  screen  being 
raised  so  as  to  cut  off  the  lower  half  of  the  beam  of  light.  Ap- 
proximately as  good  results  may  be  obtained  more  easily  by  laying 
against  the  end  of  the  tube  T  a  small  rectangular  piece  of  black 
hard  rubber  or  blackened  brass  d,  as  shown  in  the  diagram. 

Reflecting  Condenser  Ultramicroscopes  consist  of  highly 
perfected  dark-ground  illuminators  applied  to  ordinary  micro- 
scopes provided  with  special  objectives  of  low  numerical  aperture. 
In  the  special  condensers  used,  the  light  rays  are  reflected  from 
two  spherical  surfaces.  The  illuminating  rays  therefore  enter 
the  preparations  with  obliquities  greater  than  in  ordinary  dark- 
ground  illuminators  and  are  brought  to  a  correct  focus.1 

By  employing  objectives  of  low  numerical  aperture  (about 
0.85)  we  have  rays  including  only  a  low  range  of  apertures  taking 
part  in  the  formation  of  images,  although  the  illuminating  rays 
include  a  range  of  high  aperture,  1.1  to  1.35.  There  is  thus 
obtained  the  greatest  brilliancy  of  image  upon  the  darkest  of 
backgrounds. 

Although  many  different  ultracondensers  are  obtainable, 
space  forbids  a  consideration  of  more  than  two  types:  the  car- 
dioid  condenser  of  Siedentopf  as  made  by  Zeiss,  and  the  ultra- 
condenser  of  Jentzsch  as  made  by  Leitz. 

1  In  the  ordinary  paraboloid  condensers,  when  properly  constructed,  the  light 
rays  are  also  brought  to  a  focus,  but  the  focal  length  varies  from  zone  to  zone, 
hence  we  have  an  overlapping  of  images  at  the  center.  (Zeiss,  "Mikro"  Circular 
306,  p.  8.) 


66 


ELEMExMTARY   CHEMICAL   MICROSCOPY 


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ULTRAMICROSCOPES  67 

The  Cardioid  Ultramicroscope  consists  of  an  ordinary  com- 
pound microscope  M,  Fig.  30,  into  whose  substage  ring  the 
cardioid  condenser  C  is  introduced  and  held  in  place  by  the 
clamping  screw  /.  A  thin  film  of  the  liquid  to  be  examined  is 
contained  in  a  special  quartz  cell  Q  which  in  turn  is  held  in  posi- 
tion upon  the  stage  in  a  cylindrical  brass  mounting  B.  This 
mounting  may  be  leveled  or  slightly  adjusted  in  height  with 
respect  to  the  condenser  by  means  of  the  screws  S.  The  objec- 
tive O  of  the  microscope  must  be  specially  corrected  for  use 
with  the  quartz  cell  cover  and  must  have  a  numerical  aperture 
of  less  than  0.9.  This  latter  requirement  is  accomplished  by 
introducing  into  the  objective  a  funnel  diaphragm.  As  set  up 
for  use,  the  cardioid  condenser  receives  substantially  parallel 
rays  from  the  microscope  mirror  m.  The  source  of  these  rays 
must  be  some  powerful  radiant,  most  conveniently  an  arc  lamp  R. 
Parallel  rays  are  obtained  by  means  of  a  plano-convex  lens  L 
mounted  by  means  of  short  brass  bars  r,  r,  three  in  number, 
attached  to  the  metal  screen  E.  A  glass  cell  W  filled  with  water 
acts  as  a  cooling  trough.  A  black  cardboard  or  metal  diaphragm 
D  serves  to  cut  down  the  light  beam  to  the  proper  size  for  just 
filling  the  aperture  of  the  condenser.  For  convenience  in  ad- 
justment as  to  distance  and  height,  microscope,  cell  and  lens 


Q  o 

Fig.  31.     Cell  for  holding  Liquids  for  Study  with  the  Cardioid  Ultramicroscope. 

are  placed  upon  adjustable  stands  with  saddle  base  resting  upon 
an  optical  bench  of  triangular  section.  The  screen  E  is  tipped 
at  such  an  angle  as  to  project  the  rays  from  R  upon  the  properly 
inclined  mirror  m,  when  the  latter  is  at  a  distance  of  approxi- 
mately 60  centimeters  from  the  lens  L.  The  crater  of  R  should 
be  about  8  centimeters  from  L. 

The  liquid  to  be  studied  is  placed  in  a  quartz  cell  Q,  Fig.  31, 
consisting  of  a  grooved  quartz  disk  and  cover.     With  the  cover 


68  ELEMENTARY   CHEMICAL   MICROSCOPY 

in  place  the  liquid  forms  a  thin  film  q,  the  excess  of  liquid  being 
forced  into  the  groove  o.  The  quartz  cell  is  held  in  position 
upon  the  stage  of  the  microscope  by  means  of  a  brass  chamber  B 
consisting  of  a  bed-piece  into  which  the  cell  fits,  a  funnel  shaped 
section  /  pressing  gently  upon  the  quartz  cover,  and  a  top  sec- 
tion /  screwing  down  upon  the  section  /.  When  much  work  is 
to  be  done  with  this  device  it  is  best  to  have  all  the  screw  threads 
but  three  turns  cut  from  the  bed-piece  and  a  slight  recess  cut 
as  shown  at  i.  This  permits  a  rapid  removal  of  t,  and/  is  easily 
lifted  out.  As  furnished  by  the  makers/  is  flush  with  the  threads 
of  the  bed-piece  b  and  being  smooth  with  no  milling  is  hard  to 
remove.  A  small  pin  in/  fitting  into  a  hole  in  b,  not  shown  in  the 
diagram,  prevents  /  from  turning  when  t  is  being  screwed  down. 

It  is  absolutely  essential  that  both  cell  and  cover  be  absolutely 
'dean  and  free  from  all  dust  particles.  Unless  so  clean  that 
when  the  cover  is  laid  upon  the  cell  and  very  gently  pressed 
Newton's  rings  can  be  seen  the  device  is  unfit  for  use.  To  pre- 
pare Q  for  use  wash  very  thoroughly  both  pieces,  immerse  in 
hot  chromic-sulphuric  cleaning  mixture,  rinse  with  distilled 
water,  follow  by  purified  alcohol,  and  dry  in  a  current  of  warm 
air,  next  support  upon  a  loop  of  platinum  wire  and  heat  to  a 
bright  red  in  a  Bunsen  burner.  As  soon  as  the  pieces  are  cool, 
lay  in  place  in  B,  and  use  them  at  once.  When  employing  the 
quartz  cell  and  cardioid  condenser,  never  use  anything  but 
water  as  immersion  fluid  between  condenser  and  cell. 

Use  only  sufficient  liquid  to  form  a  thin  layer  q  and  not  quite 
fill  the  groove  o. 

The  objective  must  be  centered  by  means  of  the  adapter  A 
(Fig.  30),  so  that  the  bright  spot  of  light  formed  in  Q  will  fall 
in  the  center  of  the  field. 

Always  raise  or  lower  the  cardioid  condenser  so  as  to  ascertain 
the  proper  position  for  the  blackest  background  and  brightest 
diffraction  images. 

See  that  the  beam  of  light  from  the  radiant  falling  upon  the 
mirror  of  the  microscope  is  of  sufficient  diameter  to  fill  the  aper- 
ture of  the  condenser. 

Use  an  arc  of  not  less  than  15  amperes. 


ULTRAM1CR0SC0PES 


69 


In  the  absence  of  an  arc  lamp  use  a  400-watt  Mazda  lamp 
with  concentrated  filament.  Or  if  gas  alone  is  available,  employ 
an  inverted  Welsbach  incandescent  mantle  or  even  better  an 
acetylene  light. 

Be  sure  that  the  reflecting  condenser  is  high  enough  in  its 
mounting  to  just  touch  the  object  cell  upon  the  stage.  Substage 
ultracondensers  are  usually  screwed  into  their  tubular  mountings 
and  are  easily  turned  up  or  down  to  permit  of  their  accurate 
adjustment. 

The  cardioid  ultramicroscope  is  restricted  to  the  study  of 
liquids,  to  the  search  for  bacteria  not  readily  demonstrated  by 
the  paraboloid  condenser  and  to  the  examination  of  thin  textile 
fibers,  and  such  other  thin  semitransparent  and  flexible  solid 
fragments  as  will  permit  pressing  out  flat,  and  whose  thickness 
will  then  be  no  greater  than  the  thin  liquid  film  of  the  medium 
in  which  they  are  immersed. 

Cotton  and  Mouton's  Ultramicroscope1  consists  of  a  special 
prism  consisting  of  a  rectangular  prism  of  glass  having  an  in- 
clined face.  This  prism  is  laid 
upon  the  stage  of  the  microscope 
and  serves  for  the  projection  of 
an  oblique  beam  of  light  into  the 
preparation  placed  upon  its  upper 
surface.  The  diagram,  Fig.  32, 
will  make  clear  the  construction 
and  the  method  of  using.  The 
prism  P,  8  to  10  millimeters 
high,  which  converts  an  ordinary 
compound  microscope  into  an  in- 
strument for  the  study  of  ultra- 
microscopic  particles,  rests  upon  the  stage  S.  The  liquid  L,  to 
be  studied,  is  placed  upon  an  ordinary  glass  object  slide  s  and 
covered  with  a  thin  cover  glass  c.  A  drop  of  homogeneous  im- 
mersion oil  is  placed  upon  the  top  of  P,  and  the  preparation  is 

1  Cotton  et  Mouton,  C.r.,  136  (1903),  1657;  Les  Ultramicroscopes,  Paris.  1906; 
J.  Roy.  Micro.  Soc,  1903,  573;  Lemanissier,  Corps  Ultramicroscopiques,  These, 
Paris,  1905,  21. 


Fig.  32.     The  Ultramicroscope  of 
Cotton  &  Mouton. 


70  ELEMENTARY   CHEMICAL   MICROSCOPY 

carefully  laid  thereon,  avoiding  all  dust  particles  and  air  bubbles. 
This  thin  film  of  oil  O  brings  about  an  optical  homogeneity  be- 
tween prism  and  slide.  By  means  of  a  condensing  lens  C  of 
about  15  centimeters  focus  the  rays  RRR  emitted  from  an  arc 
lamp  as  radiant  are  projected  into  the  prism  through  the  in- 
clined face,  the  inclination  6  of  this  face  being  approximately 
51  degrees.  These  rays  are  totally  reflected  and  are  brought  to 
a  focus  at  the  upper  surface  of  the  glass  cover  at  the  angle  of 
total  reflection.  Any  particles  in  suspension  in  the  liquid  will 
diffract  the  light  and  diffraction  disk  images  will  be  formed  in 
the  microscope.  No  other  light  can  enter  the  instrument  and 
we  therefore  have  the  theoretical  conditions  necessary  for  the 
demonstration  of  ultramicroscopic  particles,  namely,  the  par- 
ticles become  luminous  upon  a  black  background,  the  illuminat- 
ing rays  being  of  high  aperture  while  the  image-forming  rays  are 
of  low  aperture. 

The  adjusting  of  the  illumination  in  this  device  consists  in 
ascertaining  (a)  the  proper  inclination  of  the  rays  entering  the 
prism,  and  (b)  the  correct  distance  of  C  from  P,  so  that  the  focal 
point  will  fall  in  the  proper  plane.  This  adjustment  requires 
considerable  care  and  should  first  be  undertaken  by  means  of 
some  preparation  of  a  colloidal  metal  (silver,  for  example),  and 
after  having  obtained  the  optimum  conditions  in  this  manner, 
the  preparations  to  be  studied  are  then  substituted  for  the  test 
object. 

This  type  of  ultramicroscope  is  applicable  only  to  the  examina- 
tion of  liquids.  With  proper  care  in  adjustment  it  will  yield 
results  fairly  comparable  with  the  slit  ultramicroscope. 

In  many  types  of  investigation  this  device  possesses  a  very 
desirable  feature,  namely,  that  of  permitting  at  any  time  an 
examination  of  the  preparation  by  ordinary  transmitted  light, 
for  it  is  merely  necessary  to  tip  the  mirror  of  the  micro- 
scope and  thus  send  rays  M  through  the  object  in  the  usual 
manner. 

Absolutely  clean  glass  surfaces  free  from  scratches  and  inclu- 
sions are  essential.  For  cover  glasses  the  use  of  thin  freshly- 
prepared  cleavage  films  of  clear  mica  is  suggested  by  Cotton. 


ULTRAMICROSCOPES 


71 


The  Jentzsch  Ultracondenser1  can  be  placed  upon  the  stage 
of  any  compound  microscope  and  is  so  constructed  as  to  combine 
in  itself  a  reflecting  condenser  and  cell  for  containing  liquids, 
vapors  or  gases.  It  consists,  Fig.  33,  of  a  metal  cell  M,  in  which 
are  mounted  the  two  reflecting 
glass  bodies  G,  G\  These  are  held  ■ 
in  place  by  the  cement  S,  S.  Light 
rays  enter  the  apparatus  through 
the  annular  opening  O,  strike  the 
silvered  spherical  surface  in  G,  are 
reflected  to  the  curved  sides  of  G' 
and  enter  the  central  cell  C.  The 
illuminating  rays,  therefore,  are 
substantially  at  right  angles  to  the 
optic  axis  of  the  microscope,  thus 
conforming  in  general  to  those  in 
the  slit  ultramicroscope  with,  how- 
ever, this  difference,  that  in  the 
slit  instrument  the  rays  enter  the 
cell  from  one  side  only,  while  in 
the  Jentzsch  cell  the  rays  enter  from 
all  sides  and  meet  at  the  center. 
This  instrument  may  therefore  be 
considered  as  occupying  an  intermediate  position  between  the 
slit  ultramicroscope  and  the  cardioid  type  of  ultramicroscope. 

A  cover  N  fits  into  the  mounting  M  and  is  secured  in  place  by 
a  bayonet  catch.  By  turning  the  cover  slightly  it  is  made  to 
press  down  upon  the  rubber  gasket  RR,  making  a  very  tight 
seal  against  the  upper  surface  of  G'.  The  tubes  TT  serve  for 
the  passage  of  gas  or  of  liquid  through  the  cell.  The  cover  N  is 
provided  with  a  well-like  depression  closed  at  the  end  by  the 
quartz  plate  Q.  This  well  permits  an  objective  of  long  working 
distance  to  be  focused  upon  the  particles  in  suspension  at  the 
focal  point  of  the  illuminating  rays. 

When  in  use  the  ultracondenser  is  laid  upon  the  stage  of  the 
microscope  with  the  short  tube  A  inserted  into  the  stage  opening. 

1  Made  by  Ernst  Leitz,  Wetzlar. 


Fig.  33. 


The  Jentzsch  Ultracon- 
denser. 


72  ELEMENTARY   CHEMICAL  MICROSCOPY 

The  Abbe  condenser  is  removed  or  swung  aside.  The  plane 
mirror  is  then  turned  so  as  to  reflect  a  beam  of  parallel  rays  into 
the  device.  This  beam  must  be  of  such  diameter  as  to  completely 
fill  the  aperture  of  the  condenser.  A  powerful  source  of  light  is 
essential,  preferably  an  arc  lamp  or  concentrated  filament  Mazda 
bulb.  The  mirror  is  tipped  until  the  bright  spot  of  light  appears 
at  the  center  of  the  cell.  Since  in  this  case  we  are  examining  the 
path  of  the  rays  as  in  the  slit  ultramicroscope  and  these  rays 
enter  from  all  sides  and  meet  at  the  center,  it  is  unnecessary  to 
exactly  center  the  condenser. 

Special  objectives  of  great  penetrating  power  are  necessary, 
corrected  for  the  thickness  of  the  quartz  plate  Q  and  whose 
mountings  are  of  sufficiently  small  diameter  to  permit  their 
entrance  into  the  well  in  the  cover  to  a  depth  such  that  the  focal 
point  will  lie  within  the  path  of  the  rays.  High  magnifications 
must  be  obtained  by  employing  high  power  eyepieces.  It 
follows  that  there  is  always  an  illuminated  plane  lying  below 
the  focal  plane  of  an  objective  and  a  perfectly  black  background 
is  unobtainable.  In  order  to  obtain  sharper  contrasts,  a  dia- 
phragm can  be  placed  just  above  the  mirror,  either  cutting  off 
one  side  of  the  beam  of  light  or  having  an  opening  slightly  eccen- 
tric to  that  of  the  annular  opening  in  the  ultramicroscope. 

Great  care  must  be  exercised  in  cleaning  the  cell  walls  and  the 
quartz  plate. 

For  coarse  colloids  and  for  suspended  matter  in  vapors  and 
water  the  author  has  found  this  device  of  great  convenience  and 
a  time  and  labor  saver;  but  for  very  fine  suspensions  the  results 
are  not  so  good. 

The  Immersion  Ultramicroscope.  —  In  this  instrument  de- 
vised by  Zsigmondy x  we  have  the  most  improved  type  of  micro- 
scope for  the  study  of  ultramicroscopic  particles  yet  devised; 
through  the  employment  of  immersion  objectives  of  high  numer- 
ical aperture  for  both  illumination  and  observations,  much  more 
brilliant  and  sharper  diffraction  disks  are  obtainable.  Thus  the 
existence  may  be  demonstrated  of  particles  even  smaller  than 
those  rendered  visible  by  ultramicroscopes  of  the  cardioid  type. 

1  Zsigmondy,  Physik.  Zeit.,  14  (1013),  975- 


ULTRAMICROSCOPES  73 

In  this  new  Immersion  Ultramicroscope1  both  the  illuminating 
and  observing  objectives  are  beveled  at  the  ends  so  as  to  allow 
their  front  lenses  to  be  brought  very  close  together  with  their 
axes  at  right  angles;  the  drop  of  liquid  to  be  examined  is 
placed  between  the  front  lenses,  clinging  by  capillarity.  No 
cell  is  employed.  The  light  rays  having  but  a  very  short  dis- 
tance to  travel,  even  dark  colored  liquids  may  be  studied.  Diffi- 
cultly cleanable,  expensive  cells  are  thus  wholly  eliminated,  the 
amount  of  material  required  for  study  reduced  to  a  minimum, 
and  the  images  obtained  are  exceptionally  brilliant. 

For  the  study  of  hydrosols,  water  immersion  objectives  must 
be  used,  but  for  colored  glass  and  similar  bodies  homogeneous 
immersion  objectives  are  required. 

The  construction  of  the  instrument  is  shown  in  the  diagram, 
Fig.  33A.  Fitted  to  the  body  tube  of  a  compound  microscope 
is  the  objective  carrier  C  into  which  slides  a  plate  to  which  is 
screwed  the  image-forming  objective  O.  To  the  stage  of  the 
instrument  is  attached  the  mechanism  supporting  the  illuminat- 
ing objective  I.  The  micrometer  screws  S1,  S2,  S3  provide  means 
for  the  exact  adjustment  of  the  beam  of  light  passing  in  the 
line  of  the  axis  of  the  objective  I,  so  that  it  will  fall  normal  to  the 
optic  axis  of  the  microscope.  S1  gives  an  up  and  down  adjust- 
ment, S2  forward  and  back  and  S3  from  side  to  side.  By  rack 
and  pinion  S4,  the  entire  illuminating  device  can  be  lowered  for 
cleaning,  for  the  removal  of  the  objectives,  etc.  When  raised 
in  position  for  use,  the  screw  s  is  turned,  thus  locking  the  mecha- 
nism in  place. 

The  trough  T  serves  to  catch  any  drip  when  the  liquid  is  being 
applied  between  the  objectives. 

When  in  use,  the  instrument  is  placed  on  a  bed  plate  with 
saddle  stand  upon  an  optical  bench  of  the  type  shown  in  Figs. 
24  and  30.  An  apparatus  consisting  of  a  condensing  lens  and 
an  adjustable  slit,  also  on  saddle  stands,  serves  to  throw  a 
beam  of  light  from  a  radiant  (arc  or  Nernst  lamp)  into  the 
objective  I. 

In  critical  work  the  ocular  of  the  microscope  is  furnished  with 
1  Made  by  C.  Winkel,  Gottingen,  Germany. 


74 


ELEMENTARY   CHEMICAL   MICROSCOPY 


an  adjustable  slit-diaphragm,  thus  permitting  the  cutting  down 
of  the  field  until  only  a  certain  selected  portion  is  visible. 
The  mutual  arrangement  of  the  two  objectives  is  shown  in 


Fig.  33A.     Zsigmondy  Immersion  Ultramicroscope. 


the  diagram.  These  objectives  embody  several  unique  ideas  in 
mounting,  construction  and  in  the  component  lenses  themselves; 
the  end,  or  front,  lenses  are  of  quartz.  An  examination  of  the 
diagram  will  show  that  a  drop  of  liquid  brought  into  contact 


ULTRAMICROSCOPES  75 

with  the  two  front  lenses  will  cling  in  place.  The  illuminating 
beam  will  pass  through  this  drop  in  the  focal  plane  of  the  objec- 
tive O.  The  image  resulting  upon  focusing  the  microscope  will 
appear  to  be  two  hazy  triangles  of  light  united  at  their  apices  by 
a  more  or  less  marked  brighter  thread  or  band.  In  this  band 
are  seen  the  diffraction  disks  due  to  the  infinitely  small  particles 
in  suspension.  By  means  of  the  ocular  diaphragm  all  of  the 
hazy  triangles  are  cut  off  and  the  connecting  thread  or  band  of 
light  alone  allowed  to  appear  in  the  field  of  view. 


CHAPTER  V. 

THE   EXAMINATION    OF    OPAQUE    OBJECTS,   VERTICAL 
ILLUMINATORS,    METALLURGICAL    MICROSCOPES. 

The  study  of  opaque  objects  with  ordinary  compound  micro- 
scopes requires  that  the  illuminating  rays  shall  fall  upon  the 
preparations  from  a  point  situated  above  the  stage  of  the  instru- 
ment. This  may  be  accomplished  in  several  ways:  (i)  the  rays 
from  a  radiant  can  be  projected  upon  the  surface  of  the  object 
by  means  of  mirrors,  or  by  means  of  a  condensing  lens;  (2)  a 
plate  of  glass  or  a  right-angled  prism  may  be  placed  above  the 
objective  in  a  tubular  mounting  so  as  to  fall  in  the  line  of  the 
optic  axis,  and  so  inclined  that  any  light  rays  striking  the  reflect- 
ing surface  will  be  directed  down  through  the  objective,  thus 
brightly  illuminating  the  object.  The  devices  of  Group  1 
illuminate  the  preparation  with  oblique  rays  only;  those  of  Group 
2  reflect  rays  perpendicular  to  the  surface  of  the  object  and  are 
usually  termed  vertical  illuminators.  In  the  vast  majority  of 
cases  the  examination  of  opaque  objects  through  illumination  by 
rays  normal  to  the  surface  of  the  preparation  is  preferable  to  that 
by  means  of  oblique  rays,  since  the  images  obtained  are  brighter, 
etched  figures  are  more  easily  interpreted  and  the  finer  striations 
due  to  incomplete  polishing  are  far  less  visible.  Moreover, 
photomicrographs  are  usually  more  easily  obtained. 

Formerly  parabolic  reflectors  of  silvered  glass  or  metal  attached 
to  the  objective  were  much  employed;  but  inasmuch  as  such 
devices  can  be  used  with  only  a  very  narrow  range  of  objectives, 
and  with  preparations  of  a  certain  size  only,  their  usefulness  is  so 
limited  that  chemists  have  quite  generally  abandoned  them  in 
favor  of  vertical  illuminators. 

Vertical  Illuminators  of  simple  construction  consist  of  tubular 

adapters  or  cells  so  threaded  as  to  permit  screwing  their  upper 

end  into  the  lower  end  of  the  body  tube  of  the  microscope,  and 

76  * 


VERTICAL   ILLUMINATORS,   METALLURGICAL   MICROSCOPES       77 

the  insertion  of  an  objective  into  their  lower  opening.  Mounted 
in  the  axis  of  the  adapter,  or  a  little  to  one  side,  is  a  reflecting 
device  which  receives  light  projected  upon  it  through  an  aperture 
in  the  walls  of  the  cell  and  reflects  the  rays  downward  through 
the  objective  upon  the  preparation  on  the  stage. 

The  reflecting  device  consists  of  a  totally  reflecting  prism  or  a 
thin  disk  of  glass  or  mica.  These  reflectors  are  mounted  upon 
small  metal  rods  passing  through  the  adapters  at  right  angles  to 
the  optic  axis;  a  milled  head  at  the  end  of  the  rod  permits  chang- 
ing the  angle  of  inclination  of  the  reflecting  surface. 

In  several  types  the  lateral  opening  for  the  incident  light  is 
made  variable  in  diameter  either  by  means  of  an  iris  diaphragm 
or  a  rotating  collar  provided  with  openings  of  different  sizes. 


»>^->- 


Fig.  34.    Prism  Vertical  Illuminator.         Fig.  35.    Disk  Vertical  Illuminator. 


A  typical  prism  illuminator  is  shown  diagrammatically  in  Fig. 
34.  The  reflecting  device  consists  of  a  totally  reflecting  prism  P 
so  mounted  as  to  permit  tipping  slightly  and  thus  changing  the 
direction  of  the  reflected  ray  R.  Incident  light  I  is  projected 
upon  the  prism  through  the  horizontal  opening  0.  A  diaphragm 
D  extending  not  quite  halfway  across  the  aperture  of  the  adapter 
serves  to  screen  the  prism  and  to  prevent  interfering  reflections 
from  blurring  the  image  formed  in  the  microscope. 

The  construction  of  a  disk  illuminator  is  shown  in  Fig.  35.  The 
incident  rays  I,  I  strike  a  glass  or  mica  disk  G  and  are  reflected 


78  ELEMENTARY   CHEMICAL   MICROSCOPY 

by  it  through  the  objective  attached  below.  The  rays  I,  I  enter 
through  a  circular  opening  0.  The  size  of  this  opening  may  be 
changed  by  turning  the  collar  C  which  is  provided  with  circular 
openings  of  three  different  diameters. 

Adjustment  of  Vertical  Illuminators.  —  When  the  object  to 
be  examined  is  small  and  is  supported  upon  a  glass  object  slide 
it  is  always  advisable  to  place  below  the  object  slide  a  piece  of 
black  paper,  card  or  other  dark  opaque  object,  so  that  no  trans- 
mitted light  can  enter  the  objective. 

The  size  of  the  spot  of  light  concentrated  upon  the  preparation 
should  correspond  approximately  to  the  area  of  the  preparation 
made  visible  in  the  microscope  by  the  particular  objective  em- 
ployed. It  is  therefore  desirable  that  the  diameter  of  the  bundle 
of  rays  projected  upon  the  reflecting  device  shall  be  adjustable. 
It  is  also  usually  best  that  these  incident  rays  be  nearly  parallel. 
These  two  requirements  are  met  by  interposing  between  the 
radiant  and  the  illuminator  a  suitable  lens  or  series  of  diaphragms. 
In  the  better  grades  of  illuminators,  lenses  and  diaphragms  are 
made  an  integral  part  of  the  apparatus.1 

The  source  of  incident  light  should  be  a  powerful  radiant,  as, 
for  example,  a  small  arc  lamp,  tungsten  or  Nernst  incandescent, 
or  inverted  Welsbach  gas  burner,  acetylene  light,  or  stereopticon 
lamp  with  concentration  filament,  or  better  still  a  nitrogen  filled 
tungsten.  In  all  cases  the  radiant  should  be  as  close  to  the 
illuminator  as  is  possible  for  convenience  and  safety.  With 
powerful  radiants  and  condensing  lenses,  it  is  wise  to  interpose 
between  radiant  and  illuminator  a  water  cell  of  moderate  thick- 
ness to  act  as  a  cooling  device. 

With  very  highly  polished  surfaces  the  image  obtained  is  often 
of  such  dazzling  brightness  as  to  be  almost  blinding;  in  such 
cases  a  piece  of  greenish  or  blackish  glass  should  always  be  inter- 
posed between  radiant  and  illuminator  or  placed  above  the  eye- 
piece. 

1  The  4  to  5  ampere  arc  lamps  for  microscopic  purposes  are  generally  fitted 
with  a  plano-convex  condensing  lens;  in  such  an  event  no  other  lens  between 
radiant  and  illuminator  may  be  required.  The  lamp  should  stand  8  to  12  inches 
from  the  illuminator. 


VERTICAL   ILLUMINATORS,   METALLURGICAL   MICROSCOPES       79 

Nernst  lamps  with  very  small  incandescent  filaments  often 
fail  to  yield  a  sufficiently  even  illumination;  under  such  con- 
ditions a  piece  of  ground  glass  interposed  between  lamp  and  illu- 
minator will  usually  greatly  improve  the  field  of  view,  but  will 
of  course  reduce  the  brightness  of  image. 

To  obtain  satisfactory  results  in  the  study  of  opaque  objects 
with  vertical  illuminators  it  is  important  that  the  objectives  em- 
ployed be  constructed  with  compact  mounts  and  that  the  lenses 
be  corrected  for  use  with  uncovered  objects.  Standard  microscope 
objectives  are  always  corrected  for  some  definite  cover  glass 
thickness.  Moderate  or  high  power  objectives  of  this  sort, 
therefore,  cannot  be  employed  for  the  study  of  uncovered 
preparations. 

Most  objective  manufacturers  supply  special  objectives  for 
use  with  vertical  illuminators.  Such  objectives  have  very  short 
mounts  and  have  the  rear  lens  combination  flush  with  the  upper 
edge  of  the  mount  (see  Figs.  36  and  46).  This  is  done  to  prevent 
internal  reflections  and  yields  better  fields  and  clearer  and 
brighter  images.  It  is  a  safe  rule  to  follow,  if  the  best  results  are 
wanted,  to  select  an  outfit  in  which  the  distance  between  the  re- 
flecting surface  of  the  illuminator  and  the  rear  lens  combination 
of  the  objective  is  as  small  as  possible. 

The  diagrams,  Figs.  36  and  39,  have  been  drawn  with  a  view 
of  showing  this  in  an  exaggerated  way.  In  Fig.  36  a  short  com- 
pact mount  is  shown,  the  rear  lens  combination  is  almost  in 
contact  with  the  reflecting  prism  P,  while  in  Fig.  39  an  ordinary 
objective  is  shown  and  the  distance  between  reflecting  disk  F  and 
the  rear  lens  is  so  excessive  as  will  doubtless  lead  to  interfering 
reflections  of  an  aggravated  sort.  With  the  construction  shown 
in  Fig.  39,  an  objective  with  compact  mount  would  be  essential; 

The  interior  walls  of  vertical  illuminators  must  never  be  al- 
lowed to  become  bright  but  must  be  kept  coated  at  all  times  with 
a  dull  black  finish. 

Since  the  diameter  of  the  rear  lens  combination  is  different  in 
different  objectives,  especially  when  manufactured  by  different 
firms,  it  is  evident  that  the  best  results  will  be  obtained  with 
illuminators  of  the  prism  type,  only  when  the  prism  can  be  dis- 


80  ELEMENTARY   CHEMICAL  MICROSCOPY 

placed  forward  and  back  with  reference  to  the  optic  axis  of  the 
objective  in  order  that  just  the  proper  area  of  the  objective  may- 
be covered  by  the  prism. 

When  properly  adjusted  the  image  of  the  illuminated  prepara- 
tion should  be  of  uniform  intensity  throughout  and  should  not 
have  half  the  field  hazy  and  blurred  with  a  whitish  fog.  Chang- 
ing the  distances  between  radiant,  collective  lens  and  illuminator 
and  tipping  the  prism  slightly  will  improve  matters,  but  with 
illuminators  of  the  type  shown  in  Fig.  34,  there  sometimes  re- 
mains a  slight  blurring  of  half  the  image.  To  meet  this  difficulty, 
two  sliding  diaphragms  are  provided  in  the  Zeiss  illuminator, 
which  slip  into  the  slot  S,  so  constructed  with  two  apertures  and 
a  central  opaque  stop  as  to  effectually  prevent  reflections  and 
passage  of  rays  from  the  prism  in  fine  with  the  optic  axis  of  the 
objective.  When  adjusting  the  illuminator,  first  one,  then  the 
other,  of  the  two  diaphragms  should  be  tried  to  ascertain  which 
will  yield  the  clearest  image,  observations  being  made  with  each 
diaphragm  inserted  to  different  depths;  an  exceedingly  slight 
displacement  very  seriously  affects  the  clearness  of  the  image. 

Interpretation  of  Appearances  with  Vertical  Illuminators.  — 
The  investigator  is  generally  dealing  with  more  or  less  highly 
polished  surfaces  and  with  areas,  part  of  which  are  polished,  part 
rough  and  often  studded  with  minute  bristling  points.  Less 
frequently,  as,  for  example,  in  the  study  of  material  exhibiting 
fatigue  failure,  the  preparations  are  polished  but  are  crossed  by 
exceedingly  minute  cracks  or  cleavage  planes.  To  ascertain 
whether  the  surfaces  are  polished  or  mat,  whether  we  have  to 
deal  with  elevations  or  with  depressions  and  to  enable  us  to  dem- 
onstrate slip  bands  in  fatigue  failure  requires  that  we  shall  be 
thoroughly  familiar  with  the  optic  effects  resulting  from  different 
types  of  illumination  by  reflected  light.  These  effects  have  al- 
ready been  discussed  at  length  on  pages  38  and  39,  to  which  the 
student  is  referred. 

With  ordinary  etched  metal  preparations  no  special  difficulties 
arise,  for  with  vertical  illuminators  the  polished  surfaces  appear 
bright,  the  irregular  or  mat  surfaces  more  or  less  dark.  But  to 
demonstrate  fissures,  cleavage  planes,  depressions,  etc.,  requires 


VERTICAL   ILLUMINATORS,    METALLURGICAL   MICROSCOPES       8 1 

that  the  examination  with  the  vertical  illuminator  be  supple- 
mented by  very  oblique  illumination  and  that  due  account  be 
taken  of  the  directions  of  shadows  with  respect  to  the  radiant, 
remembering  of  course  that  in  the  image  seen  in  the  microscope 
directions  are  completely  reversed. 

Polarized  Light  with  Vertical  Illuminators.  —  A  further  aid 
in  differentiating  between  the  phases  present  in  a  given  specimen 
is  afforded  by  employing  polarized  rays  for  illumination  or 
analyzing  the  light  rays  reflected  from  the  object.  The  light 
rays  reflected  from  the  polished  surfaces  of  sections  of  aniso- 
tropic crystals  are  polarized,  as  has  been  already  stated,  while 
the  rays  reflected  from  isotropic  crystal  sections  are  not  polarized. 
It  is  evident  that  if  we  pick  out  a  given  phase  and  employ  a 
magnification,  such  that  an  area  of  this  phase  alone  fills  the  field, 
we  can,  by  studying  the  nature  of  the  light  reflected  therefrom, 
often  obtain  information  of  the  greatest  value  as  to  the  nature 
of  the  composition  of  the  specimen  being  studied.  In  many 
instances  it  is  not  even  essential  to  confine  one's  attention  to  a 
selected  small  area  but  we  may  use  low  powers  which  will  include 
several  phases  in  the  image. 


Fig.  36.     Nachet  Vertical  Illuminator. 


Nachet  Vertical  Illuminator.1  —  This  instrument,  Fig.  36, 
consists  of  a  collimator  tube  C  attached  to  a  cell  F,  which  in  turn 
slips  into  the  threaded  adapter  A  and  is  held  in  place  by  the 
thumb-screw  B.  The  adapter  A  carries  at  its  upper  end  a  male 
screw  thread  of  standard  pitch,  serving  to  fasten  the  device  into 
the  end  of  the  tube  T  of  the  microscope,  while  F  is  tapped  with 

1  Manufactured  by  A.  Nachet  et  Fils,  Paris,  France. 


82  ELEMENTARY   CHEMICAL   MICROSCOPY 

standard  thread  for  the  attachment  of  the  objective  00'.  Lying 
in  the  axis  of  the  tube  C  is  the  reflecting  prism  P,  the  surface 
R  of  which  is  silvered,  and  the  outer  end  L  ground  convex,  thus 
serving  the  purpose  of  a  plano-convex  collecting  lens.  An  iris 
diaphragm  whose  diameter  is  adjustable  by  the  knob  K  is  fas- 
tened eccentrically  to  C.  The  position  of  the  center  of  the  dia- 
phragm with  respect  to  the  axis  of  C  may  be  changed  by  loosening 
the  screw  S,  thus  making  it  possible  to  alter  the  position  of  the 
point  of  incidence  upon  R  of  the  illuminating  rays  from  the 
radiant,  according  to  the  power  and  mounting  of  the  objective 
employed. 

The  light  rays  proceeding  from  the  radiant  pass  through  the 
lens  L,  and  striking  the  surface  R,  pass  through  the  objective 
which  now  acts  as  a  condenser,  throwing  a  tiny  spot  of  intense 
light  upon  the  surface  of  a  metal  preparation  M.  The  light  rays 
reflected  from  M  reenter  the  objective  to  form  the  image  seen 
in  the  microscope.  A  noteworthy  feature  of  this  type  of  vertical 
illuminator  is  the  placing  of  the  prism  P  in  such  a  position  as  to 
bring  its  lower  surface  as  close  to  the  upper  lens  combination  of 
the  objective  as  it  is  possible  to  do.  This  greatly  reduces  the 
danger  of  the  formation  of  a  hazy  or  cloudy  image  by  eliminating 
internal  reflections.  The  position  of  the  prism  P  is  fixed,  hence 
all  adjustments  of  the  light  rays  must  be  made  by  displacing  the 
iris  diaphragm  and  thus  changing  the  position  of  the  spot  of 
light  upon  the  reflecting  surface  R. 

The  Leitz  Vertical  Illuminator1  is  so  constructed  as  to  permit 
the  insertion  of  either  a  disk  or  a  right-angled  reflecting  prism 
above  the  objective,  and  is  therefore  applicable  to  all  heights  and 
powers  of  objectives. 

The  construction  is  shown  in  Fig.  37.  To  a  cylindrical  adap- 
ter K  a  collimator  tube  T  is  attached  which  carries  a  condensing 
lens  L  in  its  mounting  C.  C  slides  within  T,  thus  permitting 
regulation  of  the  diameter  of  the  illuminating  beam  of  light  pro- 
jected upon  the  reflecting  surface.  One  side  of  K  is  flattened 
and  through  this  surface  is  cut  an  opening  into  the  interior  of  the 
cell.     The  lower  part  of  this  opening  is  dovetailed  as  shown  at  d. 

1  E.  Leitz,  Wetzlar,  Germany. 


VERTICAL   ILLUMINATORS,   METALLURGICAL   MICROSCOPES        83 


< 

li|7~i 

1 

€ 

The  prism  P  and  the  disk  k  are  attached  respectively  to  the  axis 
of  the  milled  wheels  W  and  W\  These  in  turn  are  mounted  upon 
metal  plates  with  edges  obliquely  cut  so  as  to  fit  into  the  dove- 
tail d.  These  plates  when  inserted  and  pressed  in  place  are  held 
by  the  spring  s.  They  are  thus  se- 
cured in  proper  position  but  can  be 
slid  back  and  forth  in  the  slot  d. 
A  mark  S  upon  the  plates  and  an- 
other t  upon  the  adapter  serve  to 
indicate  the  proper  position  of  P  or 
k  with  respect  to  the  optic  axis  of 
the  microscope  M.  To  remove  the 
prism,  the  wheel  W  is  pressed  gently 
downwards  and  outwards,  thus  re- 
leasing the  plate  from  the  spring  s; 
W  is  then  carefully  raised  until  the 
plate  is  free  from  the  slot  d.  It 
can  then  be  removed  by  tipping  up 
slightly  and  withdrawing  from  the 
opening.  To  insert  the  disk,  turn  W 
until  the  groove  i  is  horizontal,  introduce  k  into  the  opening  and 
push  down  till  the  lower  edge  fits  into  d,  then  press  W  forward 
as  far  as  it  will  go.  The  groove  S  is  then  brought  into  coincidence 
with  L  The  reflecting  disk  k  is  fastened  to  a  mounting  by  the 
spring  fingers  v.  This  device  permits  the  rapid  and  easy  removal 
of  the  disk  for  cleaning  or  for  replacement  when  broken.  The 
objective  O  is  screwed  into  the  lower  opening  of  K;  0  in  the  illus- 
tration is  an  8  millimeter  apochromatic,  for  200  millimeters  tube 
length,  uncorrected  for  cover  glasses. 

Just  as  in  the  simple  prism  or  disk  illuminators,  the  rays  of 
light  striking  the  reflecting  surface  are  directed  downwards 
through  the  objective  upon  the  object  m. 

Parallel  light  should  fall  upon  the  lens  L.  This  is  obtained  by 
employing  a  suitable  lens  between  the  illuminator  and  radiant. 
The  Leitz  Company  supply  a  very  conveniently  mounted  lens 
for  this  purpose.  A  metal  screen  A,  Fig.  38,  is  attached  to  a 
stand  B.     Mounted  in  the  screen  is  a  lens  in  front  of  which  is  an 


Fig. 


37.     Leitz  Vertical 
Illuminator. 


84 


ELEMENTARY    CHEMICAL   MICROSCOPY 


iris  diaphragm  D.  The  stand  and  radiant  are  placed  at  such 
distances  from  L  as  to  project  a  small  beam  of  approximately 
parallel  light  upon  L.  The  milled  head  a  serves  as  a  fine  adjust- 
ment up  and  down  of  the  lens  and  diaphragm.     When  either 

daylight  illumination,  direct  sunlight,  or 
a  radiant  at  a  distance  are  to  be  used, 
the  mirrors  R2  and  Ri  are  brought  into 
service,  the  light  from  the  chosen  source 
being  received  upon  R2,  reflected  upon  Rx, 
and  thence  through  the  lens  and  dia- 
phragm opening.  When  a  radiant  close 
to  A  is  used  the  mirror  Ri  is  raised  until 
it  stands  in  a  vertical  position,  thus  giv- 
ing an  unobstructed  passage  through  the 
center  of  A. 

Correct  illumination  of  the  surface  of 
an  object  m  is  obtained  as  described 
above  by  trying  the  lens  L  at  different 
distances  from  P  and  by  tipping  P  or  k 

Fig.  38.  Condensing  Lens  until  the  most  satisfactory  angle  of  in- 
and  Iris  Diaphragm  for  clination  is  obtained.  It  may  also  be 
Use  with  Leitz  Vertical  necessary  to  slide  S  slightly  to  the  right 

or  left  of  the  indicator  t.  It  is  usually 
best  to  start  with  a  diaphragm  opening  yielding  a  beam  of  light 
which  will  not  more  than  half  fill  the  aperture  of  the  lens  L. 

Tassin  Vertical  Illuminator.  —  One  of  the  greatest  annoyances 
encountered  in  the  work  with  ordinary  vertical  illuminators  is 
the  necessity  of  readjusting  the  height  of  the  radiant  whenever  a 
change  of  objective  is  made  or  objects  of  different  thicknesses  are 
studied,  since  refocusing  is  essential  and  this  necessarily  alters 
the  position  of  the  disk  or  prism  with  reference  to  the  axis  of  the 
radiant.  To  obviate  this  defect  Tassin  has  devised  an  appara- 
tus in  which  the  radiant  —  either  a  small  tungsten  lamp  or  an 
acetylene  burner  —  is  attached  to  the  illuminator  mounting  and 
hence  in  focusing,  both  radiant  and  illuminator  are  displaced 
simultaneously  an  equal  amount;  thus  no  realignment  is  neces- 
sary.    The  construction  of  this  device  will  readily  be  understood 


VERTICAL   ILLUMINATORS,   METALLURGICAL  MICROSCOPES       85 

by  referring  to  the  diagram,  Fig.  39.  An  ordinary  disk  (or  prism) 
illuminator  I  is  attached  to  the  tube  T  of  the  microscope.  Into 
the  lower  opening  is  screwed  an  aluminum  adapter  A  which  serves 
to  hold  in  position  the  supporting  bar  B.  The  objective  O  is 
screwed  into  the  lower  end  of  A.  The  bar  B  carries  a  vertical 
sleeve  J,  fitted  with  a  thumb-screw  and  serving  to  hold  in  place 


Fig.  39.     Tassin  Vertical  Illuminator. 


the  remaining  parts  of  the  illuminator.  The  sleeve  D  carries  a 
Ramsden  eyepiece,  securely  held  in  position  by  the  screw  K. 
This  eyepiece  acts  as  a  condensing  lens.  The  correct  position 
of  the  lenses  to  obtain  a  spot  of  bright  light  of  the  requisite 
diameter  upon  the  reflecting  surfaces  is  secured  by  sliding  the 
entire  ocular  in  the  sleeve  or  by  sliding  the  lens  C  or  both.  The 
clamping  joint  E  permits  tilting  the  condenser  so  as  to  obtain 
the  correct  angle  of  incidence  upon  the  disk  or  prism.  To  ex- 
clude all  other  light  from  the  illuminator,  a  screen  S  is  attached 


86  ELEMENTARY   CHEMICAL   MICROSCOPY 

to  the  condenser  system.  Fastened  to  S  is  an  arm  G  which 
carries  the  radiant  R.  In  the  diagram  the  radiant  is  an  acety- 
lene light,  adjustable  both  up  and  down  and  forward  and  back 
in  the  mounting  H.  To  make  the  nature  of  the  burner  clearer 
the  flame  is  shown  with  its  broad  side  toward  the  condenser. 
This  is,  however,  an  incorrect  position  for  use,  the  proper  posi- 
tion being  always  with  the  edge  of  the  flame  toward  the  illuminator 
in  order  that  the  full  intensity  of  the  radiant  may  be  obtained. 
When,  instead  of  the  acetylene  burner,  a  tiny  tungsten  lamp  is 
supplied  for  use  with  this  device  a  parabolic  cover  and  reflector 
is  placed  back  of  the  bulb  and  holds  it  in  proper  place  against  the 
screen  (see  Fig.  48,  page  99).  The  light  rays  from  the  radiant 
pass  through  the  condenser  system,  strike  the  reflecting  device  of 
the  illuminator  and  are  totally  reflected  down  through  the  objec- 
tive O  upon  the  specimen  M.  The  light  rays  reflected  from  M 
pass  through  the  objective  and  strike  the  disk  F  at  an  angle  other 
than  that  of  total  reflection  and  thus  pass  through  to  form  the 
image  in  the  ocular  of  the  microscope. 

Owing  to  the  relatively  great  distance  between  the  reflecting 
disk  F  and  the  objective  it  is  essential  that  the  inner  surfaces  of 
I,  A  and  0  be  kept  a  dull  black  in  order,  to  prevent  internal 
reflections. 

The  disadvantage  of  employing  ordinary  objectives  instead  of 
those  in  special  short  mounts  will  be  apparent  at  once  from  the 
diagram,  for,  as  just  pointed  out,  the  danger  of  internal  reflec- 
tions is  very  great;  moreover,  the  length  of  I  and  A  prevent  low 
powers  from  being  employed  unless  the  microscope  is  provided 
with  a  substage  upon  which  the  specimen  can  be  supported. 
With  specimens  placed  upon  the  stage  any  attempt  to  focus  the 
upper  surface  will  entail  raising  the  body  tube  of  the  microscope 
until  the  rack  and  pinion  are  out  of  mesh. 

A  very  simple  and  efficient  device  is  shown  in  Fig.  40,  in  which 
the  radiant  moves  with  the  illuminator  in  focusing,  thus  avoid- 
ing the  necessity  of  realignment  for  different  sized  specimens  or 
changes  of  objective.  It  consists  of  a  thin,  bent  aluminum  plate 
S  inserted  between  the  body  tube  T  of  the  microscope  and  the 
vertical  illuminator  I.     Being  provided  with  a  hole  a  trifle  larger 


VERTICAL   ILLUMINATORS,   METALLURGICAL   MICROSCOPES        87 

than  the  size  of  the  illuminator  threads  it  may  be  moved  to  the 
right  or  left  for  alignment,  and  clamped  fast  by  turning  the  screw 
collar  of  I.  L  is  a  6-volt  16-candle-power  tungsten  lamp  coated 
with  "frosting  compound"  in  which  fine  graphite  has  been  sus- 
pended. This  provides  an  absolutely  opaque  covering  for  the 
lamp  and  prevents  annoying  side  lights  in  working.  A  tiny 
clear  area  is  made  in  the  coating  at  O  by  means  of  a  little  alcohol 


Fig.  40.     Vertical  Illuminator  with  Simple  Device  for  a  Tungsten 

Lamp  Radiant. 


on  a  bit  of  rag  or  cotton.  The  lamp  L  is  attached  to  the  end  of 
S  in  a  slot  cut  for  the  purpose  and  is  secured  in  place  by  wires 
soldered  to  the  terminals  of  the  lamp.  To  obtain  rigidity  these 
wires  pass  downward  through  bits  of  glass  tubing  i  as  insulators. 
S  is  so  bent  as  to  bring  the  glowing  lamp  filament  in  line  with  the 
center  of  the  illuminator  diaphragms.  As  shown  in  the  illus- 
tration the  whole  device,  including  the  coating  of  the  lamp,  can 
be  made  in  any  workshop  in  about  an  hour.  A  better  arrange- 
ment, when  shop  facilities  permit,  is  to  fasten  an  attaching 
socket  with  bayonet  catch  to  S.  The  lamp  is  connected  with 
the  usual  no-volt  lighting  circuit  with  the  interposition  of  a 
suitable  rheostat  or  lamp  bank,  allowing  the  passage  of  2  to  3 
amperes  (rive  16-candle  carbon  filament  lamps  or  two  32  and  one 


88 


ELEMENTARY    CHEMICAL   MICROSCOPY 


1 6).     Instead  of  the  16-candle-power  lamp  one  of  about  20  candle- 
power  may  be  substituted. 

The  author  has  found  tins  simple  and  inexpensive  apparatus 
very  satisfactory.  A  clear,  bright,  uniformly  illuminated  field 
is  obtained  and  there  are  no  adjustments  necessary. 


Fig.  41.     Chemical  Microscope  with  Stage  removed  and  Auxiliary  Stage  inserted 

in  the  Substage  Ring. 


Maintaining  the  Alignment  of  Radiant  and  Illuminator  may 
readily  be  accomplished  in  microscopes  provided  with  an  adjust- 
able substage  by  removing  the  condenser  or  polarizer  and  sup- 
porting the  specimen  upon  the  substage  ring.  In  the  case  of  the 
chemical  microscope,   the   stage  is  removed  by  loosening   the 


VERTICAL  ILLUMINATORS,   METALLURGICAL   MICROSCOPES        89 

centering  screws  and  lifting  out  the  stage.  An  "auxiliary" 
stage  is  then  inserted  in  the  substage  ring,  the  specimen  placed 
upon  it  and  the  focusing  is  done  by  means  of  the  substage  quick- 
acting  screw.  Delicate  focusing  may  then  be  made  by  the  fine 
adjustment  of  the  microscope.  This  method  possesses  the  ad- 
vantage of  producing  no  disturbance  of  the  alignment  of  radiant 
and  reflector  in  changing  objectives  or  in  studying  successively 
preparations  of  greatly  varying  thickness.  Fig.  41  illustrates 
the  chemical  microscope  with  auxiliary  stage  applied  for  the 
examination  of  opaque  objects.  The  auxiliary  stage  itself  is 
shown  at  A. 

Mounting  Polished  Objects.  —  In  order  to  mount  small 
preparations  for  examination  with  vertical  illuminators  so  that 
when  placed  upon  the  stage  of  the  microscope,  the  upper  or  pol- 
ished surface  will  lie  in  a  plane  at  right  angles  to  the  optic  axis 
of  the  microscope,  proceed  as  follows:  place 
upon  a  1  by  1  \  inch  extra  thick  object  slide 

of  metal  or  glass  a  small  piece  of  soft  plas-     MM 

ticine,  soft  beeswax  or  soft  paraffin;  lay  the 
object  to  be  studied  polished  side  up  upon 
the  imbedding  material  and  place  the  prepa- 
ration upon  the  substage  ring  (with  auxiliary     4|gg==^= 
stage  in  place  if  one  is  at  hand) ;  place  a  thick  „  n    "     f 

glass  object  slide  upon  the  stage  of  the  micro-  Mounting  Pieces  of 
scope  and  then  carefully  raise  the  preparation  Polished  Metal  for 
by  means  of  the  substage  screw  until  it  is      s,^udy.  with  Vertical 

iri  •  r,-  ,.,         1  Illuminators, 

pressed  firmly  against  the  object  slide,   the 

latter  being  held  in  place  with  the  fingers.     The  upper  surface 

of  the  object  to  be  studied  is  thus  made  parallel  to  the  plane  of 

the  stage  and  is  in  proper  position  for  examination  with  the 

vertical   illuminator.      Special    mounting    cells    employing    this 

same  principle  have  been  designed. 

One  of  these  cells  or  devices  is  shown  in  Fig.  42.     It  consists 

of  a  bed  plate  attached  to  a  base  and  threaded  to  carry  a  collar 

screwing  up  and  down.     The  upper  edge  of  the  collar  is  exactly 

parallel  with  the  surface  of  the  bed  plate.     The  collar  is  screwed 

up  or  down  to  accommodate  specimens  of  different  thicknesses. 


90  ELEMENTARY    CHEMICAL   MICROSCOPY 

The  specimen  to  be  mounted  is  laid  upon  a  piece  of  lens  paper, 
polished  side  down  upon  the  bed  piece.  The  collar  is  then  raised 
or  lowered  the  proper  amount  and  an  object  slip  carrying  a  bit 
of  plasticine  is  inverted  over  the  preparation  and  pressed  down 
until  each  end  touches  the  circumference  of  the  collar.  The  slip 
may  now  be  lifted  off,  carrying  with  it  the  specimen  imbedded 
in  the  plasticine  or  wax.  Laid  upon  the  stage  of  the  microscope, 
the  polished  surface  of  the  specimen  will  be  in  a  plane  normal  to 
the  optic  axis  of  the  microscope. 

Metallurgical  Microscopes.  -  -  The  extraordinary  interest  in 
the  microscopic  study  of  metals  and  alloys  within  the  last  ten 
years  and  the  astonishing  development  of  theories  relative  to 
their  constitution  and  structure,  followed  by  the  application  of 
this  information  to  the  mechanic  arts,  has  led  to  the  design  of 
special  forms  of  microscopes  to  facilitate  the  study  of  the  many 
different  problems  arising  in  the  metallurgical  industries.  In 
all  these  special  types  of  microscopes  we  have  to  deal  with 
compound  microscopes,  having  permanently  attached,  between 
ocular  and  objective,  a  vertical  illuminator,  usually  of  the  prism 
type. 

Since  the  etched  surfaces  of  metals  ordinarily  yield  images  of 
such  intricacy  that  notebook  sketches  become  impracticable, 
recourse  must  be  had  to  photography.  Most  metallurgical 
microscopes  therefore  include  as  an  integral  part  of  the  in- 
strument a  photographic  camera,  and  when  thus  provided 
they  are  often  known  as  metallographic  microscopes  or  metallo- 
graphy. 

In  order  that  the  structure  of  an  alloy  may  be  studied  it  is 
essential:  (i)  that  a  small  area  shall  be  ground  to  a  plane  surface, 
polished  and  etched;  (2)  that  this  plane  surface  shall  lie  normal 
to  the  optic  axis  of  the  microscope;  (3)  that  the  area  of  this 
plane  shall  be  so  situated  with  reference  to  surrounding  parts 
that  the  objective  may  be  brought  sufficiently  close  to  it  to 
be  focused. 

Were  the  preparation  to  be  laid  upon  the  stage  of  an  ordinary 
microscope  it  would  have  to  be  thin  and  to  have  another  sur- 
face ground  parallel   to   the  etched  surface.     To  avoid  these 


VERTICAL   ILLUMINATORS,   METALLURGICAL  MICROSCOPES        91 

difficulties  and  further  to  permit  the  examination  of  fragments 
of  moderate  size,  the  microscope  is  more  conveniently  inverted, 
i.e.,  constructed  with  the  objective  lying  below  the  stage.  The 
alloy  can  thus  be  laid  upon  the  stage,  polished  surface  down  over 
the  stage  opening.  It  will  thus  meet  the  requirement  that  its 
etched  surface  shall  lie  in  a  plane  normal  to  the  optic  axis. 
Coarse  adjustment  focusing  is  accomplished  by  displacing  the 
stage  up  or  down,  the  tube  of  the  microscope  remaining  in  a 
fixed  position,  assuring  no  disarrangement  of  the  proper  align- 
ment of  the  illuminator  with  reference  to  the  radiant. 

Most  of  the  large  metallographs  are  developments  of  the  type 
first  suggested  by  Le  Chatelier.  Two  instruments  have  been 
selected  for  illustration  as  embodying  the  largest  number  of  good 
features  to  the  exclusion  of  those  which  are  distinctly  bad. 
These  have  been  described  at  length  in  preference  to  other 
valuable  instruments  since  the  author  has  had  the  opportunity 
of  working  with  them  and  thoroughly  testing  them. 

The  Leitz  Metallurgical  Microscope.  -  -  This  instrument  con- 
sists of  the  vertical  illuminator  shown  in  Fig.  37  applied  to  a 
compound  microscope  so  arranged  as  to  lie  in  a  horizontal  posi- 
tion. The  general  arrangement  of  its  component  parts  is  illus- 
trated by  Fig.  43.  An  optical  bench  B  carries  a  series  of  stands 
with  saddle  bases.  These  stands  support  the  different  unit 
parts  of  the  instrument.  The  first  stand  carries  a  small  arc 
lamp  R,  a  condenser  C  and  a  screen  E;  attached  to  E  is  an  iris 
diaphragm  D  and  a  shutter  (not  shown  in  the  cut)  for  making 
the  photographic  exposures.  The  next  stand  supports  the  com- 
pound microscope  and  its  accessories;  the  mechanical  stage  St, 
the  illuminator  I  fitted  with  iris  diaphragm  d,  the  reflecting  prism 
V  and  the  body  tube  of  the  microscope  M  with  its  ocular;  over 
the  ocular  is  fitted  a  removable  black  glass  disk  b.  Coarse  adjust- 
ment of  the  microscope  is  obtained  by  the  wheel  F  which  raises 
and  lowers  the  stage  St  supported  by  four  pillars;  the  object  to 
be  examined  is  placed  polished  side  down  over  the  opening  of  the 
stage;  the  rays  of  light  projected  by  the  radiant  enter  the  illumi- 
nator I,  are  then  reflected  upward  through  the  objective  and 
strike  the  surface  of  the  object;    the  rays  are  thence  reflected 


92 


ELEMENTARY   CHEMICAL   MICROSCOPY 


'Eii 


H-l 


o 


VERTICAL   ILLUMINATORS,   METALLURGICAL   MICROSCOPES       93 

downward  through  the  objective  into  the  prism  V,  the  inclined 
surface  of  which  reflects  the  rays  into  a  second  right-angled  re- 
flecting prism  attached  to  the  end  of  the  tube  M.  This  latter 
prism  sends  the  rays  to  the  eye  through  the  ocular  of  the  micro- 
scope in  the  upper  end  of  M.     The  entire  tube  M  slides  into  a 


Fig.  44.     Leitz  Metallurgical  Microscope  fitted  with  Abbe  Condenser 

and  Mirror. 


sleeve:  by  pulling  out  M,  the  prism  attached  to  the  lower  end  is 
withdrawn  from  the  path  of  the  rays  which  then  pass  horizon- 
tally through  the  tube  N  carrying  at  its  end  a  projection  eyepiece. 
The  image  is  thus  formed  upon  the  ground  glass  or  photographic 
plate  of  the  camera.  A  pair  of  Hooke's  keys  K,  K  serve  to 
focus  the  image  upon  the  ground  glass.  The  rod  r  rising  from 
the  stage  serves  to  attach  a  mirror  and  an  Abbe  condenser.     By 


94  ELEMENTARY    CHEMICAL   MICROSCOPY 

removing  the  illuminator,  inserting  an  objective  directly  over 
the  prism  V,  and  attaching  an  Abbe  condenser  A  and  mirror  m 
to  the  rod  r,  the  microscope  may  be  employed  for  the  examina- 
tion of  transparent  objects  by  transmitted  light.  This  arrange- 
ment of  the  instrument  is  shown  in  Fig.  44. 

The  adjustment  of  the  illumination  in  the  Leitz  metallurgical 
microscope  is  in  every  way  similar  to  that  followed  in  the  Leitz 
vertical  illuminator  already  described.  For  high  powers  the 
makers  suggest  employing  only  the  disk  reflector,  for  moderate 
powers  the  reflecting  prism  is  used,  while  for  very  low  magnifica- 
tions a  plate  reflector  is  supplied  so  arranged  as  to  fit  between 
the  objective  and  the  preparation.  This  last  device  is  restricted 
to  such  low  magnifications,  however,  as  to  be  rarely  applicable 
to  ordinary  metallographic  studies. 

The  Reichert  Metallurgical  Microscope.  This  instrument, 
Fig.  45,  is  one  of  the  most  convenient  and  most  substantially 
built  of  its  class.  The  microscope  itself  consists  of  a  heavy  base 
A  from  which  rise  four  pillars;  the  largest  of  these,  fashioned  into 
a  handle  H,  carries  the  stage  S  provided  with  rack  and  pinion 
adjustment  for  roughly  focusing  the  preparation.  The  pillar  P 
supports  the  microscope  proper,  consisting  of  a  prism  chamber 
to  which  the  objective  O,  the  illuminator  tube  V  and  the  body 
tube  are  attached.  The  fine  adjustment  of  the  instrument  is 
accomplished  by  the  milled  screw  F.  The  prism  chamber  fur- 
ther carries  a  tube  whose  axis  is  at  right  angles  to  that  of  T, 
fitting  into  a  light  tight  sleeve  joint  in  C.  The  tube  C,  sup- 
ported by  a  pillar  provided  with  rack  and  pinion  vertical  adjust- 
ment, fits  into  the  front  of  the  photographic  camera  and  serves 
as  a  carrier  for  a  projection  eyepiece.  To  the  fourth  pillar  is 
attached  the  tube  B,  fitted  with  a  lens  for  projecting  parallel 
rays  into  V,  a  rotary  disk  provided  with  diaphragm  openings  of 
different  sizes,  a  glass  cooling  cell  W  for  water  or  for  colored 
solutions  to  be  employed  as  color  screens  and  a  slot  for  the 
insertion  of  green,  yellow  or  black  glass  slips  G  to  modify  the 
brilliancy  of  the  strongly  illuminated  surfaces.  For  greater  con- 
venience in  making  photographs  a  shutter  s  for  exposures  may 
conveniently  be  attached  to  the  end  of  the  tube  B. 


VERTICAL   ILLUMINATORS,   METALLURGICAL   MICROSCOPES       95 


96 


ELEMENTARY    CHEMICAL   MICROSCOPY 


In  the  Reichert  instrument  the  illuminating  and  image-forming 
rays  take  the  directions  indicated  in  Fig.  46.  The  light  rays  from 
a  radiant  R  enter  the  tube  V,  pass  through  a  condensing  lens 
and  are  twice  reflected  by  the  prism  P.  Passing  through  the 
objective  O,  a  spot  of  bright  light  is  formed  upon  the  surface  of 
the  inverted  preparation  M  lying  upon  the  stage  S.  The  surface 
of  M  reflects  the  rays  downward  through  O  to  the  prism  Px, 
thence  the  image-forming  rays  are  reflected  in  the  direction  E 


—  M->-E 


Fig.  46.     Path  of  Light  Rays  in  the  Reichert  Metallurgical  Microscope. 


through  the  body  tube  T  of  the  microscope  which  carries  at  its 
outer  end  an  eyepiece  of  the  usual  construction.  Owing  to  the 
space  required  for  mounting  ~Ph  the  tube  length  of  the  micro- 
scope is  greater  than  usual  and  objectives  corrected  for  a  tube 
length  of  not  less  than  200  millimeters  must  be  employed. 

The  prism  Px  is  so  mounted  that  it  can  be  rotated  through  an 
arc  of  90  degrees  by  means  of  the  milled  head  K.  To  K  is  at- 
tached an  indicator  I  which  marks  the  position  of  the  reflecting 
surface.  In  the  position  of  I  shown  in  the  diagram  the  image 
formed  by  the  objective  is  sent  to  the  observing  tube  of  the 
microscope;  turned  through  90  degrees  the  rays  are  reflected 
into  the  tube  indicated  by  the  dotted  circle  C,  whose  axis  is  at 
right  angles  to  that  of  T.  The  tube  C  is  fitted  with  a  projection 
ocular  whose  function  it  is  to  form  an  image  upon  the  ground 


VERTICAL   ILLUMINATORS,   METALLURGICAL   MICROSCOPES       97 

glass  or  photographic  plate  of  the  camera.  Set  screws  H  H  serve 
to  adjust  and  to  limit  the  amount  of  rotation  of  K  so  that  when 
turned  as  far  as  it  will  go  in  either  direction  the  images  formed 
will  be  comprised  in  circular  fields  uniformly  illuminated  through- 
out their  entire  areas. 

The  illuminator  prism  P  is  so  mounted  as  to  permit  its  hori- 
zontal displacement  below  the  objective  in  order  to  conform  to 
the  requirements  of  different  objectives.  Were  P  immovable, 
in  the  majority  of  cases  half  the  image  formed  in  the  eyepiece 
would  be  covered  with  a  whitish  cloud  or  fog.  The  amount  of 
horizontal  displacement  is  also  such  that  P  can  be  slid  entirely 
beyond  the  aperture  of  the  objective,  thus  allowing  an  unob- 
structed passage  of  rays  from  the  objective  to  Pi.  Transparent 
objects  placed  upon  the  stage  can  thus  be  studied  by  transmitted 
axial  light  or  by  oblique  light. 

To  prevent  undue  strains  upon  the  stage  when  large  specimens 
must  be  examined  a  supporting  rod  L  rises  from  the  pillar  to 
which  C  is  attached  and  passes  through  a  clamp  attached  to  the 
pillar  support  of  C.  Tightening  a  set  screw  in  the  clamp  pre- 
vents any  vertical  movement  on  that  side  of  the  stage  and  as  a 
further  protection,  a  set  screw  is  also  provided  for  locking  the 
coarse  adjustment  in  position.  The  stage  may  thus  be  held 
rigidly  in  any  plane  and  relatively  heavy  objects  may  safely  be 
laid  upon  it. 

The  mirror  m  is  attached  to  a  bar  of  such  length  that  it  can  be 
swung  close  to  the  objective  and  serves  to  project  oblique  light 
upon  the  surface  of  the  preparation  in  the  study  of  fissures,  slip 
bands,  cavities,  etc. 

The  radiant  R  consists  of  an  arc  lamp,  that  shown  in  the  figure 
being  a  "hand  feed"  type.  The  body  of  the  lamp  is  raised  upon 
its  supporting  rod  until  the  center  of  the  crater  of  the  arc  lies  in 
the  line  of  the  optic  axis  of  the  lenses  mounted  in  B  and  V. 
Hooke's  keys  Ni,  N2,  N3  permit  adjustment  of  alignment  and 
illumination  while  looking  into  the  instrument,  Ni  turning  the 
lamp  from  side  to  side,  N3  up  or  down,  and  N2  approaching  or 
drawing  apart  the  carbons.  In  this  lamp  both  +  and  —  car- 
bons are  of  the  same  size,  but  the  pitch  of  the  screw  threads 


98  ELEMENTARY   CHEMICAL   MICROSCOPY 

feeding  and  moving  them  are  in  the  ratio  2:1.  Hence  the  move- 
ment through  the  key  N2  compensates  for  the  more  rapid  con- 
sumption of  the  horizontal  carbon. 

The  projection  eyepiece  in  C  forms  an  image  upon  the  ground 
glass  or  photographic  plate  of  the  camera  U,  the  size  of  the  photo- 
graph taken  being  regulated  by  extending  or  contracting  the 
bellows  of  the  camera.  A  graduated  scale  engraved  upon  the 
optical  bench,  upon  which  the  camera  slides,  permits  a  record 
being  kept  of  the  position  of  the  photographic  plate  at  the  time 
of  the  exposure.  In  order  to  obviate  the  necessity  of  passing  to 
the  rear  of  the  camera  to  look  at  the  ground  glass  and  to  focus, 
a  mirror  is  placed  within  the  camera  box  and  hinged  to  one  cor- 
ner; by  means  of  the  lever  /  this  mirror  may  be  swung  diagonally 
across  the  box  at  an  angle  of  45  degrees.  The  image  of  the 
preparation  will  thus  be  projected  upon  this  mirror  whence  it  is 
reflected  upon  the  glass  g.  The  investigator  can  thus  examine 
the  image  and  focus  the  same  without  leaving  his  seat  before  the 
body  tube  T.  When  an  exposure  is  to  be  made,  the  lever  /  is 
pushed  back  until  the  mirror  lies  flat  against  the  side  of  the 
camera.  There  is  thus  an  unobstructed  passage  through  C  and 
the  camera  to  the  photographic  plate. 

Although  the  Reichert  metallurgical  microscope  is  one  of  the 
best  and  most  convenient  of  its  kind,  it  has  at  least  one  very 
serious  defect.  It  is  supplied  with  specially  constructed  objec- 
tives whose  mounts  are  of  small  diameter  so  made  as  to  drop 
into  a  sleeve  or  adapter  instead  of  screwing  in  as  is  the  case  with 
ordinary  objectives.  The  purchaser  of  the  instrument  must 
therefore  obtain  his  entire  outfit  of  objectives  at  the  time  the 
microscope  is  bought.  If  future  purchases  of  objectives  are 
required  it  is  necessary  to  obtain  them  from  Reichert  and  in 
order  to  be  certain  of  their  proper  centering  the  microscope 
should  be  sent  to  factory.1 

1  This  difficulty  has  been  eliminated  in  the  instrument  in  the  author's  laboratory 
by  fitting  the  opening  above  the  illuminating  prism  with  standard  or  international 
thread  ("society  screw"),  thus  permitting  the  use  of  all  standard  objectives  irre- 
spective of  the  firm  manufacturing  them,  and  thus  greatly  increasing  the  usefulness 
of  the  microscope,  particularly  when  the  instrument  is  employed  for  the  examina 
tion  of  transparent  objects  by  transmitted  light. 


VERTICAL  ILLUMINATORS,   METALLURGICAL   MICROSCOPES       99 


Metallurgical  Microscopes  for  the  Examination  of  Large 
Castings,  etc.,  are  now  manufactured  by  a  number  of  different 
firms.     Such    instruments    are    often    designated,    as    "  Works 

Microscopes,"  since  their  purpose  is 
the  study  of  materials  of  construction 
already  in  place  or  too  large  to  bring 
into  the  laboratory. 


Fig.  47.     Stead  Works  Micro- 
scope. 


Fig.  48.     Tassin  Metallurgical  Microscope. 


As  indicated  by  the  name  and  purpose  they  are  compact,  sub- 
stantially built  and  easily  transportable.  They  consist  essen- 
tially of  a  compound  microscope,  whose  pillar  or  handle  arm  has 
been  separated  from  the  remainder  of  the  instrument  in  a  line 
in  the  plane  of  the  stage,  and  attached  to  a  suitable  base  or  to 
three  legs.  In  other  words,  these  instruments  are  microscopes 
without  stage  or  substage.  When  in  use,  the  base  rests  upon 
the  object  to  be  studied  and  the  tube  carrying  objective,  illumi- 
nator and  ocular  is  raked  down  until  the  surface  of  the  object 
is  in  focus,  there  being  an  aperture  in  the  base  in  line  with  the 
optic  axis  or  the  base  is  provided  with  widely  divergent  legs. 
Figs.  47,  48  and  49  illustrate  typical  instruments  of  this  class. 


IOO 


ELEMENTARY   CHEMICAL   MICROSCOPY 


In  the  Stead  instrument,  Fig.  47,  the  body  tube  is  supported 
upon  three  adjustable  legs.  Focusing  is  done  by  hand  by  rais- 
ing or  lowering  the  tube  in  a  sleeve.  When  in  focus  the  instru- 
ment is  held  in  place  by  a  clamping  screw  C.  A  vertical  illumi- 
nator of  the  disk  type  forms  an  integral  part  of  the  instrument.1 
The  radiant  in  this  case  consists  of  a  tiny  incandescent  electric 
lamp  enclosed  in  a  sleeve  at  right  angles  to  the  illuminator  mount- 
ing. As  the  instrument  is  intended  for  low  magnifications  only, 
no  fine  adjustment  is  provided. 

A  somewhat  similar  idea  in  illum- 
inator construction  is  found  in  the 
Tassin  metallurgical  microscope.2 
In  this  instrument,  Fig.  48,  we  find 
the  illuminator  of  the  form  already 


Fig.  49.     Leitz  Metallurgical  Microscope. 

described  on  page  84,  Fig.  39,  the  radiant  being  either  an  elec- 
tric or  an  acetylene  lamp.  The  microscope  itself  has  no  substage 
but  is  mounted  upon  a  heavy  base  with  central  opening  and  pro- 
vided with  four  large  leveling  screws. 

The  third  type  of  instrument  is  illustrated  by  the  Leitz  metal- 
lurgical microscope,  Fig.  49.     Here  we  have  a  compound  micro- 

1  See  Stead,  Work  Shop  Microscopes.     J.  Roy.  Micro.  Soc.  1909,  20,  22. 

2  For  its  application  see  Tassin,  The  Microstructure  of  Steel  Castings,  J.  Ind. 
Eng.  Chem.,  5  (1913),  713.  Metallography  as  Applied  to  Inspection,  J.  Ind. 
Eng.  Chem.,  6  (1914),  95- 


VERTICAL   ILLUMINATORS,   METALLURGICAL   MICROSCOPES     IOI 

scope,  consisting,  as  usual,  of  stage  and  substage,  but  with  this 
difference,  the  tube  and  pillar  are  detachable  from  the  stage,  and 
the  substage  and  support  detachable  from  the  base.  By  attach- 
ing the  microscope  and  pillar  to  the  base  there  is  obtained  a 
works  microscope  applicable  to  the  study  of  large  castings.  The 
area  of  the  casting  to  be  studied  is  visible  in  the  microscope  in 
the  opening  between  the  legs  of  the  horse  shoe  base.  Light  from 
a  suitable  radiant  is  deflected  by  the  mirror  m  into  a  right-angled 
prism  attached  to  the  end  of  the  illuminator. 

For  the  proper  illumination  of  the  objects,  the  methods  and 
precautions  already  described  on  pages  78  to  82  are  obviously, 
equally  applicable. 


CHAPTER  VI. 


USEFUL    MICROSCOPE   ACCESSORIES,    LABORATORY 
EQUIPMENT,    WORK   TABLES,    RADIANTS. 

Drawing  Cameras  (Camera  Lucidas).  —  It  is  very  frequently 
the  case  that  sketches,  relative  proportions  of  structural  details, 
or  actual  measurements  of  component  parts  of  preparations 
being  studied  must  be  entered  into  notebooks.  Free-hand  draw- 
ing is  tedious,  difficult,  and  if  a  sketch  to  scale  is  required,  as  is 
usually  the  case,  an  exceptionally  good  judgment  of  proportion 
is  essential.  To  obviate  these  difficulties  a  drawing  camera  may 
be  employed.  Although  there  are  many  types  of  these  devices 
upon  the  market,  the  chemist  is  usually  restricted  to  those  forms 

which  permit  employing  the  micro- 
scope in  a  vertical  position. 

The  most  convenient  of  these 
drawing  cameras  are  shown  in  Figs. 
50  and  51. 

If,  after  attaching  one  of  these 
devices  to  the  tube  of  the  micro- 
scope above  the  ocular,  the  worker 
looks  into  the  instrument,  he  is 
able  to  see  simultaneously  both  the 
preparation  and  the  page  of  the  notebook. 

In  the  forms  shown  in  Figs.  50  and  51,  known  as  Abbe  prism 
camera  lucidas,  there  is  placed  above  the  ocular  a  cube  of  glass 
which  has  been  cut  diagonally,  the  surface  of  one-half  being 
silvered  and  cemented  again  in  place,  after  a  central  oval  per- 
foration has  been  made  through  the  silvered  surface.  This  oval 
aperture  allows  the  image-forming  rays  of  the  microscope  to  reach 
the  eye  while  the  silvered  surface  reflects  from  a  mirror  the  image 
of  the  notebook  page  or  drawing  paper.  Fig.  52  shows  diagram- 
matically  the  path  of  the  light  rays,  the  dotted  lines  indicating 

102 


Fig.  50.     Small  Abbe  Drawing 

Camera. 
(Bausch  &  Lomb  Optical  Co.) 


USEFUL   MICROSCOPE  ACCESSORIES 


103 


Fig.  51.    Large  Abbe  Drawing  Camera.     (Spencer  Lens  Co.) 


Fig.  52.     Diagram  of  the  Path  of  Light  Rays  in  Abbe  Drawing  Cameras 


104  ELEMENTARY   CHEMICAL  MICROSCOPY 

the  image-forming  rays  from  the  drawing  paper  BB  reflected  by 
the  mirror  M  to  the  reflecting  surface  ef  of  the  Abbe  prism  P, 
and  thence  to  the  eye  of  the  observer.  The  solid  lines  indicate 
the  image-forming  rays  from  the  preparation  upon  the  stage  of  the 
microscope,  passing  through  the  aperture  in  ef  also  reaching 
the  eye.  It  is  obvious  that  the  observer  is  able  to  see  both  the 
image  of  the  preparation  and  the  drawing  paper  and  can  there- 
fore trace  upon  the  paper  with  a  pencil  the  outlines  and  many 
details  of  structure  of  the  preparation. 

In  order  to  avoid  distortion  of  the  drawing  the  mirror  M  must 
be  so  inclined  that  the  light  ray  be  shall  fall  normal  to  the  paper. 

From  an  examination  of  the  diagram  it  will  be  seen  that  unless 
the  opening  in  ef  is  placed  at  the  eye-point  considerable  light  will 
be  lost  and  unsatisfactory  results  will  be  obtained.  Before  at- 
taching a  drawing  camera  always  first  ascertain  the  position  of 
the  eye-point  (see  page  13).  It  not  infrequently  happens  that  in 
designing  an  ocular,  the  manufacturer  fails  to  take  into  account 
the  fact  that  the  investigator  may  wish  to  use  a  drawing  camera. 
The  eye-point  may  in  such  cases  lie  so  close  to  the  eye  lens  or 
may  He  so  far  above  it  as  to  render  the  employment  of  an  Abbe 
prism  camera  impracticable.  Because  of  this  great  difference  in 
the  relative  position  of  the  eye-point  in  different  oculars  it  is  best, 
in  purchasing  an  Abbe  camera,  to  select  one  of  the  type  shown 
in  Fig.  51,  since  in  instruments  of  this  sort  the  prism  mounting 
is  of  the  smallest  dimensions  possible  and  the  distance  between 
prism  and  clamping  ring  will  allow  exceedingly  great  latitude  in 
movements  up  and  down. 

In  order  to  equalize  the  light  intensity  reaching  the  eye  from 
preparation  and  drawing  paper,  a  series  of  dark  glasses  of  graded 
degrees  are  mounted  so  as  to  turn  and  be  swung  in  position,  by  a 
ring  between  prism  and  paper,  and  a  ring  between  prism  and 
ocular.  By  properly  adjusting  the  diaphragm  of  the  Abbe  con- 
denser and  then  selecting  the  right  glasses  in  these  rings,  it  is 
always  possible  to  obtain  a  clear  image  of  both  preparation  and 
drawing  pencil. 

The  large  cameras  of  the  type  just  referred  to,  are  provided 
with  a  graduated  extension  bar  to  which  the  mirror  is  attached 


USEFUL   MICROSCOPE  ACCESSORIES 


io5 


to  facilitate  adjustments,  and  the  axis  upon  which  the  mirror 
tips  is  graduated  into  degrees.  When  the  paper  lies  horizontally 
with  respect  to  the  optic  axis  of  the  microscope,  the  mirror  should 
be  set  at  45  degrees,  providing  that  the  mirror  bar  is  long  enough 
to  prevent  interferences  due  to  a  reflected  image  of  the  stage; 
if  not,  then  the  mirror  must  be  tipped  to  an  angle  nearer  to  the 
horizontal  and  the  drawing  paper  inclined  until  the  central  rays 
become  normal  to  it.  The  amount  of  inclination  of  the  drawing 
surface  must  be  twice  as  many  degrees  as  the  mirror  is  tipped 
below  45. 

Camera  lucidas  serve  not  only  for  drawing  but  are  most  useful 
in  micrometry,  in  reading  thermometers  when  melting,  boiling 
or  subliming  points  are  determined,  or 
in  reading  scales  of  small  voltmeters  or 
ammeters  when  observations  are  being 
made,  for  upon  looking  into  the  micro- 
scope both  the  preparation  and  the 
scale  of  the  instrument  may  be  seen. 

The  Leitz  Drawing  Eyepiece,  shown 
in  section  in  Fig.  53,  consists  of  a  neg- 
ative eyepiece  whose  lenses  are  so 
mounted  as  to  permit  the  insertion  of 
a  reflecting  prism  P  just  above  the  eye 
lens  extending  to  the  optic  axis  of  the 
ocular.  Light  rays  (as  indicated  by 
the  dotted  line)  from  the  drawing  paper 
enter  the  prism,  are  twice  totally  re- 
flected from  the  inclined  surfaces  of 
the  prism  and  enter  the  eye  together 
with  the  image-forming  rays  of  the  microscope.  The  eye  there- 
fore perceives  the  image  of  the  object  under  the  microscope 
apparently  projected  upon  the  drawing  paper.  Neutral  tinted 
glasses  N  serve  to  reduce  the  light  intensity  from  the  drawing 
paper  and  to  thus  facilitate  following  the  tracings  of  the  pencil 
point.  The  screw  S  serves  to  clamp  the  device  in  place  while 
in  use. 

Two  types  of  these  Drawing  Eyepieces  are  manufactured, 


Fig.  53.     Drawing  Eyepiece. 
(E.  Leitz.) 


106  ELEMENTARY   CHEMICAL   MICROSCOPY 

one  for  use  with  the  microscope  in  a  vertical  position,  the  other 
for  a  slightly  inclined  instrument. 

Since  the  prism  forms  an  integral  part  of  the  eyepiece,  changes 
in  magnification  must  be  made  wholly  by  changing  objectives  or 
changing  the  distance  from  drawing  board  to  prism. 

Microspectroscopes  or  Spectroscopic  Oculars  consist  of  direct 
vision  spectroscopes  as  integral  parts  of  microscope  eyepieces. 
They  are  usually  constructed  after  the  Sorby-Browning  pattern, 
using  a  compound  direct  vision  Amici  prism.  These  prisms 
consist  of  either  three  or  five  units,  a  prism  of  flint  glass  between 
two  of  crown  glass,  or  two  prisms  of  flint  glass  alternating  with 
three  of  crown  glass.  This  prism  is  mounted  just  above  the  eye 
lens  of  the  ocular,  while  the  slit  of  the  spectroscope  is  placed  in 
the  plane  of  the  diaphragm  of  the  eyepiece.  Usually  a  com- 
paring prism  is  provided,  which,  when  in  position,  cuts  off  half 
the  width  of  the  spectrum  and  permits  placing  in  juxtaposition 
with  the  spectrum  of  the  material  being  studied,  the  absorption 
spectrum  of  a  solution  of  known  composition.  The  position  of 
bands  or  the  amount  of  the  spectrum  cut  off  is  determined  by  an 
arbitrary  scale;  or  by  means  of  an  Angstrom  scale  reading  in 
wave  lengths,  projected  upon  the  spectrum,  or  by  means  of  some 
indicating  device  moving  the  length  of  the  spectrum,  its  position 
at  any  given  point  being  indicated  by  a  scale  moved  by  a  microm- 
eter screw.  This  last  type  is  the  only  one  of  value  to  the 
chemist. 

The  microspectroscope  illustrated,1  Figs.  54  and  55,  is  pro- 
vided with  a  measuring  device  capable  of  yielding  concordant 
measurements  with  a  very  fair  degree  of  accuracy.  The  instru-' 
ment  consists  of  the  cell  or  chamber  K  in  which  are  housed  the 
slit  s,  the  comparing  prism  p,  a  movable  diaphragm  d,  and  in  the 
lower  opening  the  field  lens  /  of  the  ocular.  A  small  opening  O 
in  the  side  of  K  permits  light,  reflected  by  the  mirror  m,  to  enter 
the  prism  p  and  thus  yield  a  spectrum  in  juxtaposition  to  that 
obtained  from  the  object  under  the  microscope.  The  solution 
or  transparent  solid  used  for  comparison  is  held  before  the  open- 
ing O  by  means  of  the  clamps  CC.     The  knob  P  serves  to  swing 

1  Manufactured  by  W.  &  H.  Seibert,  Wetzlar,  Germany. 


USEFUL  MICROSCOPE  ACCESSORIES 


107 


the  comparing  prism  p  beneath  the  slit  or  out  to  one  side.  T 
attached  to  a  right  and  left  threaded  spindle  serves  to  widen  or 
narrow  the  slit  s.  Attached  to  the  upper  part  of  K  is  the  re- 
mainder of  the  eyepiece  with  its  eye  lens  e  vertically  movable  by 
rack  and  pinion  through  the  milled  head  F.     Fitting  above  e  is 


Fig.  54.     Microspectroscope.     (W.  &  H.  Seibert.) 


a  tube  A  carrying  an  Amici  prism  R  consisting  of  three  prisms  of 
crown  glass  (no  =  1.534)  alternating  with  two  prisms  of  flint 
glass  (nD  =  1.587). 

Since  the  total  deviation  of  a  ray  of  light  entering  a  series  of 
prisms  is  equivalent  to  the  sum  of  the  deviations  which  would  be 
imparted  to  it  by  each  unit  in  turn,  it  follows  from  the  alternate 


io8 


ELEMENTARY   CHEMICAL  MICROSCOPY 


Fig.  55.     Microspectroscope. 


arrangement  of  the  glass  prisms,  three  low  and  two  high,  that  the 
deviation  of  the  system  will  be  the  difference  between  the  devia- 
tions produced  by  the  crown  and  flint  prisms.  The  net  result  is 
that  for  rays  of  medium  wave  length  (yellow-green)  the  path  of 
the  emerging  rays  lies  substantially  in  the  same  line  as  that  of  the 


USEFUL   MICROSCOPE   ACCESSORIES  109 

rays  entering  the  system,  hence  it  is  usual  to  term  such  a  prism 
system,  a  direct  vision  prism.  The  dispersive  power  of  such  a 
system  is  equivalent  to  that  which  would  be  produced  by  the 
prisms  of  flint  glass  alone.  In  the  diagram,  Fig.  55,  the  total 
dispersion  indicated  is  therefore  not  theoretically  correct. 

The  measuring  device  of  the  Seibert  microspectroscope  fits 
above  the  tube  A.  It  consists  of  a  diaphragm  with  a  very  tiny 
triangular  opening  I  mounted  in  the  sliding  plate  B  and  illumi- 
nated by  the  mirror  n;  an  image  of  this  opening  is  projected  by 
the  lens  /  as  a  tiny  bright  white  triangle  upon  the  inclined  surface 
of  the  prism  R  and  is  then  reflected  to  the  eye  at  i.  The  knob  L 
serves  to  slide  the  lens  /  and  thus  focus  the  image  of  the  triangular 
opening.  The  plate  in  which  the  diaphragm  is  mounted  can  be 
displaced  vertically  by  means  of  a  micrometer  screw;  the  amount 
of  displacement  is  indicated  upon  the  scale  S  and  by  the  gradua- 
tions upon  the  drum  g;  one  complete  rotation  of  the  drum  (100 
divisions)  is  equivalent  to  one  division  of  the  scale  S. 

To  facilitate  the  illumination  of  the  diaphragm  opening  I,  the 
mirror  n  is  attached  to  a  rotating  collar  /. 

The  position  of  a  line  in  the  spectrum  is  ascertained  by  bring- 
ing the  triangle  image  to  such  a  position  that  the  line  bisects  the 
vertical  angle.  The  scale  and  drum  divisions  are  then  read  and 
recorded.  The  equivalent  of  this  reading  in  wave  lengths  is 
obtained  from  the  calibration  of  the  instrument  by  the  method 
given  below. 

Should  the  object,  whose  absorption  spectrum  is  to  be  studied, 
be  so  small  that  its  image  fails  to  completely  fill  the  length  of  the 
slit,  the  slit  must  be  shortened  until  the  object  completely  fills  it 
and  there  will  be  no  light  reaching  the  eye  which  does  not  first 
pass  through  the  object.  This  is  accomplished  by  pushing  the 
comparing  prism  into  place,  thus  cutting  the  spectrum  in  half. 
At  the  same  time  the  mirror  m  is  turned  aside  so  that  no  light 
enters  O.  Should  the  image  of  the  object  still  fail  to  fill  the 
length  of  the  slit,  the  sliding  diaphragm  d  is  moved  toward  the 
center  by  turning  the  head  D,  until  the  slit  length  is  reduced  to 
the  proper  dimensions. 

In  order  to  center  the  object,  examine  and  focus  it,  it  is  neces- 


IIO  ELEMENTARY   CHEMICAL   MICROSCOPY 

sary  to  remove  the  tube  A  carrying  the  prism.1  The  slit  5  is 
opened  to  its  full  width  and  the  microscope  focused  in  the  usual 
manner,  the  eyepiece  having  first  been  itself  focused  by  means 
of  F  and  set  at  the  proper  calibration  reference  mark  c. 

Before  the  instrument  can  yield  scale  readings  convertible 
into  wave  lengths,  it  must  be  calibrated.  This  will  necessitate 
placing  upon  its  tubes  certain  reference  or  indicator  marks. 
The  instrument  is  removed  from  the  microscope  tube  M,  pointed 
toward  the  sky  and  the  slit  narrowed.  The  spectrum  should 
appear  as  a  long  rectangular  band  of  colored  light  crossed  by 
many  fine  black  lines  at  right  angles  (Fraunhofer's  lines)  to  its 
length.  Should  these  lines  appear  inclined,  the  tube  A  must 
be  turned  slightly  until  they  are  made  normal  to  the  spectrum 
length.  Having  thus  carefully  adjusted  the  prism  to  the  proper 
position  with  reference  to  the  slit,  make  the  reference  marks  b 
upon  A  and  upon  r  in  order  to  fix  this  position.  Now  carefully 
focus  the  spectrum  by  means  of  F,  using  the  narrowest  slit 
possible  until  the  Fraunhofer  lines  appear  sharpest.  This  should 
be  done  on  a  bright  sunny  day.  Scratch  the  mark  c  to  indicate 
this  position.  Turn  /  and  tip  the  mirror  n  so  as  to  reflect  light 
into  the  tube  and  move  L  until  a  bright  sharp  white  triangle 
is  seen  when  looking  into  the  eyepiece.  Carefully  turn  the  cap 
carrying  the  measuring  device  until  the  apex  of  the  bright  tri- 
angle takes  a  position  just  a  trifle  above  the  center  of  the  spec- 
trum band.  This  position  is  easily  ascertained  by  pushing  the 
comparing  prism  in  place  beneath  the  slit;  half  the  spectrum  will 
now  disappear.  The  most  convenient  position  for  the  bright 
spot  of  light  is  when  the  base  of  the  triangle  falls  just  below  the 
dividing  line.  Make  the  marks  indicated  at  a  so  as  to  fix  this 
position.  The  instrument  is  now  ready  for  calibration.  It  can 
be  taken  apart  at  any  time  and  the  parts  replaced  so  as  not  to 
alter  the  values  of  the  scale  divisions.  After  calibration,  if,  at 
any  future  time,  wave  length  measurements  are  required,  the 

1  In  other  forms  of  microspectroscopes,  as,  for  example,  those  manufactured  by 
Zeiss,  Leitz  and  others,  the  Amici  prism  is  so  mounted  as  to  swing  upon  a  hinge 
above  the  eye  lens.  This  greatly  simplifies  adjustments.  Unfortunately  all  of 
these  instruments  have  measuring  devices  too  crude  to  be  of  value  to  the  chemist. 


USEFUL   MICROSCOPE   ACCESSORIES 


III 


instrument  is  first  set  so  that  all  the  reference  marks  take  the 
same  positions  as  when  the  spectroscope  was  first  adjusted. 

Measurements  of  line  or  band  positions  are  made  by  bringing 
the  bright  white  triangle  to  such  a  position  that  the  line  or  the 
edge  of  the  band  bisects  the  acute  angle  of  the  triangle.  The 
scale  S  and  drum  g  are  then  read  and  recorded.  S  reads  from  o 
to  10,  g  in  hundredths  of  S.  For  example,  in  the  instrument 
illustrated:  Fraunhofer  c  =  0.42,  D  =  1.41,  G  =  7.1 1,  etc. 

In  calibrating  by  means  of  the  Fraunhofer  lines  direct  sunlight 
should  be  thrown  into  the  instrument  by  means  of  the  microscope 
mirror.  For  bright  lines,  hold  the  instrument  clamped  securely 
in  place  on  a  suitable  clamp  stand  and  direct  it  toward  a  Bunsen 
burner  flame  into  which  the  metallic  salts  are  to  be  introduced. 
The  following  lines  will  be  found  convenient  for  the  calibration : 


Line. 


A.. 
Ka. 

a.  . 


B.. 
Lia- 
C. 


Na  (D). 
Baa 

Tl 

E 

bi 

b2 


Corresponding 

wave  length  in 

Angstrom  units. 


7600 
7682 
7201 
6870 
6708 
6563 
5893 
5535 
535° 
5270 

5i83 
5173 


Line. 


F... 

Sr/3. 

Cs«. 
CS0. 

d... 
G.  . 

g- 
Rb/3 
Rb« 
h... 

H,. 


Corresponding 

wave  length  in 

Angstrom  units. 


4681 
4607 
4555 
4593 
4383 
4308 
4226 

4215 
4202 
4103 
3968 


When  only  approximate  results  in  terms  of  wave  lengths  are 
needed,  a  very  convenient  device  consists  in  plotting  the  curve 
for  the  spectroscope  upon  coordinate  paper,  using  wave  lengths 
as  ordinates  and  scale  divisions  as  abscissas.  Such  a  calibration 
curve  is  shown  in  Fig.  56,  the  black  dots  indicating  the  measure- 
ments actually  made. 

For  the  study  of  the  absorption  bands  of  liquids  under  the 
microspectroscope,  the  most  convenient  cells  will  be  found  to  be 
tubes  of  different  size  bores  and  lengths  whose  ends  are  ground 
true  at  right  angles  to  their  axes.  A  piece  of  compact  cork  pro- 
vided with  a  central  orifice  is  cemented  to  a  glass  object  slide  by 


112 


ELEMENTARY   CHEMICAL  MICROSCOPY 


means  of  shellac  or  balsam.  The  short  pieces  of  tube  fit  snugly 
into  the  hole  in  the  cork  and  are  pressed  tightly  against  the  object 
slide.     The  tubes  are  thus  easily  removable  and  readily  cleaned. 


800 
90 
80 
70 
60 
50 
40 
30 
20 
10 

700 
90 
80 
70 
60 
50 
40 
30 

— 

\ 

\ 

20 

10 

600 

.<  90 

n  80 

3  70 

g'eo 
S  50 

o40 
>  30 

^10 
500 
90 
80 
70 
60 
50 
40 

20 

10 

400 

90 
80 
70 
60 
350 

lllllllll 

lllllllll 

lllllllll 

lllllllll 

IHIIIIH 

lllllllll 

lllllllll 

HIIHIII 

lllllllll 

0123456789  10 

Scale  Divisions 

Fig.  56.     Calibration  Curve  of  a  Seibert  Microspectroscope. 

The  position  of  maximum  intensity  of  an  absorption  band 
should  always  be  determined  by  observing  the  situation  of  the 
vanishing  point  of  the  band  after  repeated  dilutions. 

It  should  be  borne  in  mind  that  the  position  of  a  band  may 
be  changed  greatly  through  increased  or  diminished  dissociation, 
and  that  the  absorption  bands  given  by  a  crystal  may  be  quite 


USEFUL   MICROSCOPE   ACCESSORIES 


n3 


different  from  those  given  by  the  same  material  in  solution  and 
furthermore  that  the  absorption  spectra  are  usually  different  in 
different  directions  through  the  crystal. 

Mechanical  Stages.  —  In  order  to  facilitate  moving  objects 
and  to  ensure  certainty  in  covering  a  given  area  in  quantitative 
work  some  form  of  device  permitting  accurate  coordinate  move- 
ments in  the  plane  of  the  stage  becomes  essential.  Such  devices 
are  known  as  mechanical  stages  and  are  indispensable  in  a  great 
variety  of  microscopic  work.  Microscopes  with  a  fixed  mechani- 
cal stage  are  not  desirable  for  ordinary  chemical  laboratory  in- 
vestigations, owing  to  the  danger  of  spilling  corrosive  liquids. 


Fig.  57.     Attachable  Mechanical  Stage. 


Attachable  mechanical  stages  are  far  better  for  our  purposes. 
These  stages  are  of  many  forms  though  in  principle  and  manner 
of  employment  all  are  similar.  A  type  applicable  to  the  chemical 
microscope  (Fig.  3)  is  shown  in  Fig.  57.  The  large  arm  A  en- 
circles the  pillar  of  the  microscope  and  is  held  firmly  in  place  by 
the  set  screw  S,  seating  into  a  shallow  slot  made  in  the  base  of  the 
pillar.     This  ensures  replacing  the  stage  each  time  in  exactly 


114  ELEMENTARY    CHEMICAL   MICROSCOPY 

the  same  position.  Coordinate  movements  are  obtained  by  the 
milled  wheels  T,  T.  The  graduated  scales  in  each  instance  are 
supplied  with  verniers  ?,  v.  The  object  slide  is  held  in  position 
by  the  fingers  F,  /;  a  spiral  spring  in  the  joint  of  F  presses  it 
firmly  against  the  corner  of  the  slide.  The  screws  a,  a  permit 
changing  the  distance  between  F  and  /,  thus  providing  for  the 
use  of  object  slides  or  cells  of  different  sizes. 

By  means  of  the  mechanical  stage  the  investigator  is  enabled 
to  search  systematically  the  entire  area  of  a  preparation  in  such 
a  manner  as  to  ensure  that  no  portion  has  been  missed,  nor  has 
any  portion  been  twice  examined,  a  matter  of  vital  importance 
in  quantitative  work,  in  clinical  microscopy  and  in  the  examina- 
tion of  foods  for  adulteration. 

Before  attaching  a  mechanical  stage  of  this  type  to  the  micro- 
scope, first  lay  a  thin  card'or  a  piece  of  thick  paper  upon  the  stage 
of  the  microscope,  then  lay  the  mechanical  stage  upon  the  paper 
and  securely  clamp  it  in  place  about  the  base  of  the  pillar  of  the 
microscope.  Pull  out  the  card  or  paper  and  the  stage  is  ready 
for  use.  The  card  or  paper  has  as  its  function  preventing  the 
arms  from  rubbing  upon  the  stage  when  the  arms  of  the  mechan- 
ical stage  are  moved.  Unless  a  tiny  space  is  left  between  the 
microscope  stage  and  the  mechanical  stage,  a  free  and  smooth 
movement  of  the  preparation  back  and  forth  beneath  the  objec- 
tive may  be  seriously  hampered. 

In  order  that  full  use  may  be  made  of  a  mechanical  stage  the 
amount  of  displacement  must  be  indicated  by  equivalent  scales 
on  each  of  the  two  movements.  It  is  therefore  essential  to  find 
the  value  and  the  uniformity  of  the  scale  divisions  and  to  find  the 
diameter  of  the  field  of  the  microscope  as  indicated  on  the  scale  of 
the  stage.  This  may  be  accomplished  by  laying  a  stage  microm- 
eter in  place  between  the  clips  F/  of  the  stage  and  measuring 
the  displacement  under  a  cross-haired  eyepiece  for  different  por- 
tions of  each  of  the  lateral  scales  of  the  stage.  There  is  thus 
ascertained  the  true  value  of  the  graduations,  whether  both 
scales  are  equivalent  and  whether  the  scale  divisions  are  of  uni- 
form size.  To  determine  the  amount  of  stage  displacement 
necessary  to  just  include  an  entirely  new  area,  bring  a  line  of 


USEFUL   MICROSCOPE   ACCESSORIES 


XI5 


the  stage  micrometer  just  tangent  to  the  circle  of  the  field  of 
view,  read  the  stage  and  displace  the  micrometer  until  the  same 
line  is  tangent  to  the  field  at  the  opposite  end  of  the  diameter 
of  the  field-circle  and  again  read  the  stage  scale.  The  difference 
in  the  readings  will  give  the  number  of  scale  divisions  necessary 
to  bring  an  entirely  new  area  of  the  preparation  into  the  field 
of  view  with  that  particular  optical  combination  which  has  been 
employed. 

When  the  entire  area  of  the  preparation  must  be  studied,  the 
student  must  of  course  look  into  the  instrument  while  the  prep- 
aration is  slowly  displaced  in  one  direction,  as,  for  example,  to 
the  right  or  left,  and  then  turn  the  stage  up  or  down  the  proper 
number  of  scale  divisions  and  again  observe  the  slowly  changing 
field  as  it  is  displaced  in  the  opposite  direction  to  the  left  or 
right. 

Rotating  or  Orienting  Devices.  —  It  not  infrequently  happens 
that  irregular  fragments  of  material  must  be  carefully  studied, 
or  that  the  exact  relation  of  one  surface  to  that  adjacent  to  it 
must  be  determined,  or  that  the  behavior  of  light  rays  sent 
through  the  body  in  different  directions  be  ascertained.  To 
facilitate  the  changing  of  the  position  of  the  substance  and  to 
enable  the  worker  to  so  place  it  that  the  surface  being  examined 
shall  lie  in  a  plane  normal  to  the  optic  axis,  various  orienting 
devices  have  been  suggested. 

The  simplest  of  these  consist  of  either  metal  or  glass  hemi- 
spheres of  such  a  size  as  to  fit  into  the  opening  of  the  stage  or  into 
the  opening  of  a  plate  laid  upon  the  stage;  the  upper  part  of  the 
hemisphere  is  usually  a  truncated  cone.  Having  a  lower  hemi- 
spherical surface  the  apparatus  may  be  tipped  in  any  direction 
and  at  any  angle  up  to  approximately  45  degrees. 

The  Glass  Hemisphere,  as  employed  simply  for  the  purpose 
of  facilitating  the  examination  of  irregular  objects,  is  shown  in 
Fig.  58;  a  band  around  the  hemisphere  gg  is  rough  ground  so  as 
to  prevent  slipping  when  the  device  is  tipped.  The  object  0  laid 
upon  the  upper  or  flat  surface  can  be  so  tipped  as  to  permit  the 
different  surfaces  to  be  studied  without  difficulty. 

In  certain  classes  of  microscopes  as,  for  example,  Dennstedt's 


n6 


ELEMENTARY   CHEMICAL  MICROSCOPY 


''Universal"  microscope1  the  stage  itself  consists  of  a  huge 
hemisphere,  thus  permitting  the  orientation  of  irregular  objects 
in  all  directions.  This  microscope  was  designed  to  meet  the 
requirements  of  forensic  investigations  where  large  objects  of 
irregular  outline  are  the  rule. 


Fig.  58.     Large  Glass  Hemisphere.     An  Accessory  which  greatly  facilitates  the 

Study  of  Irregular  Objects. 


The  application  of  the  hemisphere  is  also  found  in  several  mi- 
croscopes intended  for  the  study  of  metals.  Here,  however,  we 
are  dealing  with  opaque  objects,  and  needing  reflected  light  only, 
the  orienting  device  can  be  constructed  entirely  of  metal.  A 
good  example  of  this  style  of  construction  is  found  in  Robin's 
metallograph.2 

In  this  instrument  the  stage  is  attached,  Fig.  59,  to  the  micro- 
scope stand  by  a  ball-and-socket  joint  as  shown,  making  it  pos- 
sible to  focus  upon  any  given  area  of  very  irregular  specimens. 

To  facilitate  the  examination  of  crystals  with  reference  to  their 
different  behavior  toward  polarized  light  according  to  the  direc- 
tion through  them  that  the  light  is  sent,  Schroeder  van  der  Kolk 
suggested  fastening  the  specimens  to  a  small  glass  hemisphere. 

1  Dennstedt,  Die  Chemie  in  der  Rechtspflege,  p.  285,  Leipzig,  1910. 
*  Robin,  Traite1  de  Metallographie,  p.  50,  Paris,  1912. 


USEFUL   MICROSCOPE   ACCESSORIES 


II7 


This  idea  was  later  elaborated  by  E.  ten  Siethoff 1  who  combined 
the  hemisphere  with  a  system  of  condensing  lenses,  thus  permit- 
ting not  only  the  orientation  of  a  crystal  and  its  study  under  the 
influence  of  plane  polarized  light  sent  through  in  the  directions 
of  the  different  axes  of  vibration  but  also  permitting  observations 
with  strongly  converging  polarized  light  in  different  positions. 
The  apparatus  consists  of  a  condenser  which  is  laid  upon  the  stage 
of  the  microscope,  the  diameter  of  its  mounting  being  such  as  to 
fit  into  the  stage  opening.  The  construction  is  shown  in  Fig.  60. 
The  crystal  fragment  is  laid  upon  the  flat  surface  of  the  glass 
hemisphere  S. 


Fig.  59.  Robin  Ball-and-socket  Stage 
for  Metallurgical  Microscopes. 


Fig.  60.     ten   Siethoff   Glass  Hemi- 
sphere. 


Fig.   6r.     Klein  Orienting  Apparatus. 


Klein's  Orienting  Apparatus,2  Fig.  61,  consists  of  a  glass  cell  C 
to  which  a  conical  tube  T  is  attached  into  which  is  ground  a  plug 

1  Central,  f.  Min.,  1903,  657. 

2  Manufactured  by  Voight  and  Hochgesang,  Gottingen,  Germany. 


Il8  ELEMENTARY   CHEMICAL   MICROSCOPY 

or  stopper  S.  To  the  outer  end  of  this  stopper  is  fastened  a 
metal  head  M,  whose  circumference  is  graduated,  each  division 
being  equal  to  two  degrees.  These  graduations  are  not  intended 
for  accurate  measurement  but  merely  to  serve  as  a  guide  in  rotat- 
ing the  material  cemented  to  the  knob  k  at  the  inner  end  of  S. 
For  use  the  cell  and  stopper  are  placed  in  a  metal  mounting  B 
and  laid  upon  the  stage  of  the  microscope.  The  cell  C  is  filled 
with  a  liquid  of  such  refractive  index  as  to  practically  obliterate 
the  usual  heavy  black  contour  bands.  Leakage  is  prevented  by 
holding  the  stopper  tightly  in  place  by  the  tension  spring  /.  A 
■curved  linger,  fastened  by  the  screw  A,  holds  the  glass  parts  in 
the  metal  mounting  and  allows  easy  removal  for  cleaning.  An 
index  mark  i  upon  the  tube  T  furnishes  a  means  of  determining 
the  amount  of  rotation  of  the  object  attached  to  k.  The  instru- 
ment is  provided  with  two  cells,  one  10  millimeters  deep  and  one 
15  millimeters  deep,  and  a  special  condensing  lens  K  for  observa- 
tions with  converging  polarized  light. 

A  Simple  Device  for  Orientation,  often  perfectly  satisfactory, 
consists  in  cementing  the  object  to  the  point  of  a  needle  or 
tiny  glass  rod  and  inserting  the  other  end  of  the  needle  or  rod 
into  a  mass  of  plasticine.  The  needle  or  rod  can  be  moved  in 
any  direction  and  secured  in  place  by  gentle  pressure  of  the 
lingers  upon  the  plasticine.  Solid  angles  of  tiny  crystals  may 
thus  be  computed  by  measuring  the  plane  angles  of  the  different 
faces  in  turn.1 

Lens  Holders.  —  Frequently  low  magnifications  are  required 
in  preparing  or  separating  material  for  microscopic  study,  but 
placing  the  objects  upon  the  stage  of  the  compound  microscope 
is  inconvenient  or  impossible.  Recourse  may  then  be  had  to 
magnifiers  held  in  some  sort  of  easily  adjustable  stand.  The 
author  has  found  a  stand  of  the  general  style  shown  in  Fig.  62 2 
to  be  the  most  useful.  The  lens  holder  itself,  consisting  of  a 
spring  clip  C,  renders  the  stand  applicable  to  a  wide  variety  of 
Uses  other  than  merely  supporting  lenses.  The  hinged  arms  and 
thumb-screw  admit  of  adaptation   to  any  position  and  to  all 

1  Kley,  Rec.  trav.  chim.  Pays-Bas.,  19  (1900),  13. 

2  Made  by  the  Bausch  &  Lomb  Optical  Co.,  Rochester,  N.  Y. 


USEFUL   MICROSCOPE   ACCESSORIES 


119 


angles  and  elevations.  The  rack  and  pinion  serves  as  a  fine 
adjustment  or  to  facilitate  the  examination  of  the  surfaces  of 
irregular  objects. 


Fig.  62.     Lens  Holder. 


Reagent  Containers.  —  Dry  reagents  for  microchemical  analy- 
sis are  conveniently  kept  in  tiny  glass  stoppered  vials  in  a 
block  of  wood  (Fig.  63),  the  stoppers  of  which  are  numbered  or 
lettered  and  the  contents  recorded  upon  a  small  chart  which  may 


Fig.  63.     Reagent  Set  for  Microchemical  Analysis. 

be  placed  under  the  glass  plate  on  the  work  table.  A  transport- 
able set  of  reagents  is  shown  in  Fig.  64,  modeled  after  the  reagent 
box  designed  by  Behrens,1  differing  from  that  of  Behrens  in  only 
two  particulars;  in  having  upright  stoppers  in  the  vials  instead 

1  Anleitung  z.  Mikrochem.  Anal.,  Leipzig,  1899,  P-  29- 


120 


ELEMENTARY   CHEMICAL  MICROSCOPY 


of  flat  mushroom  form,  thus  permitting  the  removal  of  stoppers 
or  vials  more  quickly  and  easily,  and  in  having  all  the  vials 
glass  stoppered  instead  of  half  of  them  with  rubber  stoppers. 


Fig.  64.     Reagent  Set  for  Microchemical  Analysis.     (Behrens.) 

The  common  acids,  such  as  hydrochloric,  nitric,  sulphuric  and 
acetic,  in  daily  use  may  be  kept  in  small  bottles  provided  with 
pipettes,  Fig.  65.  In  similar  bottles  distilled  water,  dilute 
ammonia  and  dilute  glycerine  may  be  placed.  A  tiny  shallow 
tray  will  be  found  convenient  for  holding  the  set  of  liquid  reagents. 
Small  bottles  holding  liquid  reagents  must  frequently  be  emptied 
and  filled  with  fresh  material,  owing  to  the  extraction  of  soluble 
constituents  from  the  glass  walls  of  the  containers. 

Ammonium  fluoride  and  other  fluorine  compounds  are  placed 
in  small  stoppered  tubes  made  of  hard  rubber,  Fig.  66,  or  in 
cerosine  lined  vials.  In  the  latter  case  frequent  renewing  of  the 
reagent  is  essential. 


USEFUL   MICROSCOPE  ACCESSORIES 


121 


V 


Fig.  65.     Reagent  Bottle  with 
Barnes  Pipette.     (X  f.) 


Fig.  66.     Ebonite  Tubes  for  Ammonium 
Fluoride. 


Glass  Rods  and  Pipettes.  —  The  tiny  amounts  of  reagents 
required  for  microchemical  tests  are  most  conveniently  removed 
from  bottles  and  vials  by  means  of  drawn-out  glass  rods  or 
by  platinum  wires  mounted  in  a  glass  handle.  The  type  of  glass 
rod  found  to  be  most  useful  is  shown  in  Fig.  67 ;  if  one  or  two 
millimeters  of  the  drawn-out  end  are  slightly  roughened  with  a 
piece  of  fine  carborundum  or  emery  cloth,  or  ground  on  a  wheel, 


Fig.  67.     Drawn-out  Glass  Rod  and  Platinum  Wire  for  handling  Reagents. 

it  will  be  found  that  both  liquids  and  solids  are  more  easily  trans- 
ferred and  handled  than  if  the  glass  be  smooth.  Slightly  breath- 
ing on  the  end  of  the  rod,  or  touching  it  to  one's  fingers  before 
bringing  it  in  contact  with  the  reagent  will  cause  tiny  fragments 
of  dry  powders  to  cling  to  the  rod  long  enough  to  permit  all  usual 
transfers.  Similarly,  roughening  the  end  of  the  platinum  wire 
improves  its  carrying  power.  Rods  and  wires  roughened,  neces- 
sarily require  more  care  in  cleaning  after  use  than  when  polished. 


122 


ELEMENTARY   CHEMICAL   MICROSCOPY 


Tiny  pipettes  may  be  employed  for  transferring  solutions  or 
liquid  reagents,  but  are  so  difficult  to  keep  thoroughly  clean  that 
it  is  wiser  to  employ  short  lengths  of  tubing  of  capillary  bore  made 
by  drawing  out  odds  and  ends  of  glass  tubing.  Such  substitutes 
for  pipettes  draw  up  the  solutions  to  which  they  are  touched  by 
capillarity  --  the  liquid  can  easily  be  expelled  by  gently  blowing 
into  one  end  of  the  tube,  the  other  end  being  held  against  an 
object  slide.  After  transferring  the  liquid,  the  capillary  tube  is 
thrown  away. 

Spatulas.  -  -  Larger  amounts  of  dry  reagents  than  can  con- 
veniently be  handled  by  the  glass  rods  or  platinum  wires  may  be 
transferred  by  means  of  small  platinum  spatulas,  Fig.  68,  made 


Fig.  68.     Platinum  Spatula  for  Microchemical  Analysis.     (Full  size.) 

from  a  piece  of  platinum  wire  about  one  millimeter  in  diameter 
and  8o  to  85  millimeters  long,  one  end  of  which  is  hammered  out 
flat  on  a  polished  steel  surface  until  it  becomes  a  little  over  3 
millimeters  wide  and  the  flattened  surface  about  10  millimeters 
long.  The  blade  thus  prepared  is  shaped  and  smoothed  with  a 
fine  file  and  polished.  The  end  of  the  handle  is  given  a  gentle 
blow  or  two  with  a  hammer,  filed  to  a  double  chisel  edge  and 
polished,  thus  giving  an  instrument  useful  in  breaking  up  small 
fragments  of  soft  salts,  or  in  loosening  reagents  in  the  set  of  vials 
referred  to  above. 


Fig.  69.     Forceps  for  Microscopic  Work.     (Full  size.) 

Forceps.  -  -  For  picking  up  tiny  fragments  of  dry  material, 
handling  cover  glasses,  small  watch  glasses,  etc.,  forceps  (Fig. 
69)  with  fine  curved  tips  are  indispensable.  The  corrugations 
usually  found  on  the  points  should  be  carefully  filed  away  until 
the  tips  are  almost  smooth. 


USEFUL   MICROSCOPE   ACCESSORIES  123 

When  deliquescent  or  corrosive  materials  are  to  be  handled 
the  forceps  should  be  provided  with  solid  platinum  tips,  Fig.  70. 
No  microchemical  outfit  can  be  considered  as  complete  without 
platinum  tipped  forceps.  Just  as  in  the  case  above  cited  the 
roughening  at  the  tips  should  be  carefully  removed  and  at  least 


Fig.  70.     Forceps  with  Platinum  Tips.     (Full  size.) 

one  of  the  tips  also  filed  flat  and  smooth  on  the  outside,  thus  al- 
lowing the  tip  to  be  used  as  a  tiny  spatula.  Tips  should  be 
sufficiently  stiff  and  rigid  to  permit  holding  fragments  firmly  to 
obviate  all  danger  of  dropping  material  or  bending  the  tips. 
Foil-like  tips  are  for  this  reason  an  abomination  since  the  slightest 
excess  of  pressure  causes  them  to  bend  and  loosen. 

Object  Slides  and  Other  Supports.  —  Object  slides  or  slips 
employed  in  microchemical  analysis  should  be  from  1  to  1.5 
millimeters  thick  and  made  from  glass  of  such  composition  as  to 
be  as  resistant  as  possible  to  the  action  of  solvents.  The  color- 
less glass  object  slides  in  common  use  in  America,  so  excellent 
for  ordinary  microscopic  work,  are  easily  attacked  by  all  the 
usual  solvents  and  reagents  employed  in  qualitative  analysis. 
Great  care,  therefore,  is  necessary  when  very  minute  amounts 
of  material  are  to  be  tested,  to  avoid  being  led  into  serious  error 
arising  from  the  extraction  of  constituents  from  the  glass  slides. 

Object  slides  of  greenish  glass,  the  usual  material  supplied 
some  years  ago,  and  sometimes  still  found  on  sale,  are  much 
better,  being  harder  and  more  resistant  to  the  action  of  chemicals. 

Standard  slides,  3  inches  by  1  inch,  are  too  long  and  should 
be  cut  in  half,  or  half-size  slides  purchased,  since  microchemi- 
cal reactions  are  generally  performed  at  the  corners  of  the 
slides,  seldom  if  ever  at  the  center.  A  full-sized  slide  cannot  be 
satisfactorily  rotated  on  the  stage  of  the  polarizing  microscope 
with  the  material  situated  at  one  corner,  since  the  slide  extends 
too  far  beyond  the  rim  of  the  stage;  nor  can  material  be  heated 
at  the  center  of  the  slide  without  incurring  the  danger  of  breaking. 

Object  slides  of  ordinary  non-resistant  glass  rapidly  become 


124  ELEMENTARY    CHEMICAL   MICROSCOPY 

etched,  corroded  and  scratched  and  should  then  be  discarded. 
Before  being  used,  new  slides  should  be  dropped  in  warm  chromic 
acid  cleaning  mixture,  washed  free  from  all  acid  in  hot  distilled 
water,  drained,  dried,  in  a  locality  free  from  dust,  and  when  dry 
stored  in  covered  boxes  or  wide-mouthed  bottles.  The  simplest 
test  which  can  be  applied  to  an  object  slide  to  determine  its  fit- 
ness for  use  is  to  place  upon  its  surface  a  small  drop  of  distilled 
water  and  slowly  tip  the  slide;  if  the  drop  flows  readily  across, 
leaving  an  unbroken  streak  of  water,  the  surface  is  clean;  if, 
however,  the  drop  refuses  to  flow  or  if  upon  flowing  it  immedi- 
ately breaks  away  from  the  mother  drop,  the  surface  of  the  slide 
is  dirty  or  greasy  and  is  not  fit  for  microchemical  manipulations. 
Passing  a  greasy  object  slide  slowly  through  the  flame  of  a  Bun- 
sen  burner  will  often  render  it  fit  for  use. 

All  cloths  used  in  wiping  slides,  etc.,  must  be  free  from  lint 
and  washed  absolutely  free  from  starch,  dextrine  or  other  fillers 
which  may  have  been  present.  The  so-called  "glass- toweling" 
of  commerce,  after  thorough  washing,  will  be  found  to  be  one  of 
the  best  materials  for  use.  It  must  be  remembered,  however, 
that  after  handling,  any  such  material  takes  up  sufficient  greasy 
material  from  the  hands  as  to  render  it  unfit  for  use;  for  this 
reason  it  will  be  found  convenient  to  have  the  toweling  cut  in 
short  lengths  and  to  mark  one  side  in  some  manner  and  always 
apply  the  hands  to  the  marked  side  only.  Frequent  laundering 
is  essential. 

The  difficulty  of  preparing  absolutely  clean  slides  is  never 
fully  appreciated  until  one  has  tried  working  with  dark-ground 
illuminators  and  various  types  of  the  ultramicroscope.  In  the 
more  refined  methods  of  ultramicroscopic  investigations  it  is 
found  that  glass  slides  cannot  be  made  sufficiently  free  from 
objectionable  surface  films  for  use,  and  recourse  must  be  had  to 
quartz  slides  or  disks  which,  after  cleaning  as  described  above, 
are  heated  to  bright  redness  just  prior  to  being  employed. 

Quartz  (fused  silica)  slides  may  now  be  obtained  from  any 
firm  dealing  in  this  material,  sufficiently  free  from  air  bubbles, 
to  permit  using  even  high  powers,  and  of  such  transparency  as 
to  leave  little  to  be  desired.     Small  slips  or  tiny  cells  of  silica 


USEFUL   MICROSCOPE   ACCESSORIES  125 

will  be  found  most  useful  where  corrosive  acid  chemicals  are 
employed  or  where  the  material  must  be  heated  to  a  temperature 
somewhat  higher  than  the  fusing  point  of  glass.  In  the  investi- 
gation of  ultramicroscopic  particles  or  in  observations  upon  the 
action  of  ultraviolet  light,  fused  silica  supports  and  covers  are 
essential.  The  price  of  silica  object  slides  is  still  so  high,  however, 
as  to  be  prohibitive  to  their  employment  save  in  investigations 
where  glass  or  platinum  foil  cannot  possibly  be  used. 

For  use  with  hydrofluoric  acid  and  its  salts  object  slides  of  thin 
celluloid  will  be  found  practicable  and  far  more  convenient  than 
glass  slides  varnished  or  coated  with  Canada  balsam.  In  the 
absence  of  good  celluloid  slips,  glass  object  slides  may  be  coated 
with  a  thin  film  of  "Zapon"  or  "Bakelite"  varnish.1  Although 
celluloid  may  now  be  obtained  sufficiently  clear  and  colorless  for 
all  the  usual  microchemical  methods  involving  tests  with  fluor- 
ides it  possesses  the  drawback  of  great  inflammability  and  since 
most  of  these  tests  require  a  gentle  heat  for  their  proper  develop- 
ment, exceeding  great  care  is  necessary  to  avoid  the  complete 
destruction  of  the  slide  and  preparation  during  heat  treatments. 
Object  slides  made  from  "fireproof"  photographic  films  of  cellu- 
lose acetate  are  therefore  better  than  slips  of  ordinary  celluloid 
and  it  is  to  be  regretted  that  sheets  made  from  cellulose  acetate 
cannot  be  purchased  in  the  open  market  of  the  same  thickness 
as  those  made  from  the  nitrocellulose. 

Treatment  of  material  with  alkalies  or  at  a  high  heat  must 
be  confined  to  supporting  slips  made  from  platinum  foil.  In 
fact,  a  small  piece  of  platinum  foil  15  to  20  mm.  long  by  about 
7  mm.  wide,  sufficiently  thick  to  remain  flat  when  heated  at  a 
corner  may  be  considered  as  a  necessity.  The  foil  must  be 
kept  flat,  clean  and  polished.  Since  it  is  opaque,  the  materials 
must  eventually  be  transferred  to  glass,  quartz,  or  celluloid 
slides  for  examination  after  having  been  subjected  to  the  proper 
reagent  or  heat  treatment.  When  very  low  magnifications  are 
permissible  it  is  possible  to  examine  the  material  upon  the  plati- 
num foil  without  transferring,  the  illumination  being  either  by 
oblique  light  or  by  some  form  of  vertical  illuminator. 

1  See  also  page  269. 


126 


ELEMENTARY   CHEMICAL   MICROSCOPY 


Watch  Glasses.  -  When  volumes  of  liquid  greater  than  can 
be  handled  upon  object  slides  become  necessary,  small  watch 
glasses  10  millimeters  and  25  millimeters  in  diameter  will  be  found 
convenient.  Only  the  deep  type  of  watch  glass  should  be 
employed;  for  example,  a  25  millimeter  watch  glass  should  be 
from  3  to  5  millimeters  deep.  Instead  of  10  millimeter  shallow 
watch  glasses,  object  slides  with  a  depression  ground  into  them 
will  be  found  better  and  more  convenient. 

Watch  glasses  are  useful  for  covering  preparations,  for  making 
tiny  moist  chambers,  for  microdesiccators,  for  distilling  and 
subliming,  and  for  evaporating  solutions  to  small  bulk. 

Most  small  watch  glasses  are  made 
from  soft  non-resistant  glass,  a  fact 
which  should  be  borne  in  mind  when 
using  them. 

Still   larger  volumes  of  liquid  than 
can  be  accommodated  in  small  watch 
glasses  are  best  concentrated  in  small 
Fig.  71.    Best  Form  of  Glass  evaporators  of   transparent  quartz  or 

or    Quartz    Evaporator    for    T  ,  /„.  N       Tr  ,,  ..,    a 

Microchemical  Work.  Jena  glaSS  (FlS"  7*)-     If  those  ™^  flat 

bottoms  are  chosen  they  may  be  placed 
upon  the  stage  of  the  microscope  and  any  crystals,  deposits,  etc., 
examined  with  low  powers  as  well  as  if  the  material  were  trans- 
ferred to  a  glass  object  slide. 

Gas  Lamps  for  Microchemical  Work.  —  The  form  of  "mi- 
crochemical burner"  commonly  referred  to  in  the  older  manuals 
on  the  microscope  and  microscopic  methods  is  shown  in  Fig. 
72.  This  burner  answers  admirably  for  all  purposes  involving 
only  moderate  heating  of  very  small  amounts  of  material. 
Since,  however,  microchemical  methods  often  require  a  prelimi- 
nary handling  of  several  grams  or  cubic  centimeters  of  substance, 
the  burner  shown  in  Fig.  73  will  be  found  to  afford  a  wider  range 
of  usefulness.  It  also  occupies  less  space  upon  the  work  table. 
It  consists  of  an  ordinary  Bunsen  burner  provided  with  a  side 
tube  for  a  "reserve"  or  "pilot"  flame.  In  the  form  illustrated, 
the  tiny  flame  B  (reserve  flame)  employed  for  microchemical 
work  is  furnished  by  a  small  brass  tube  inside  the  Bunsen  tube. 


USEFUL  MICROSCOPE   ACCESSORIES 


127 


This  flame  is  always  burning  when  the  gas  is  turned  on  at  the 
gas  main;  its  height  is  regulated  by  the  screw  S  so  as  to  be  from 
3  to  4  millimeters  high.  If,  as  often  happens,  this  tiny  flame 
cannot  be  lowered  to  the  proper  size,  remove  the  screw  S,  and 
drop  into  the  hole  a  small  fragment  of  very  soft  annealed  copper 


Fig.  72.     Burner  for  Microchemical 
Analysis. 


Fig.   73.     Burner  for  Microchemical 
Analysis.     ( X  ^.) 


wire,  replace  the  screw  and  turn  until  the  copper  fragment  has 
been  crushed  sufficiently  to  partially  obstruct  the  flow  of  gas. 
Turning  the  stopcock  A  lights  the  large  burner  and  serves  to 
regulate  the  size  of  the  Bunsen  flame.  The  burner  is  not  sold 
with  the  ring  R,  as  shown  in  the  figure,  but  this  attachment  can 
be  made  in  a  few  minutes  by  fastening  a  bent  copper  or  brass 
wire  to  a  split  brass  ring  which  may  be  raised  or  lowered  and 
maintains  its  position  through  friction,  or,  if  possible,  a  heavier 


128 


ELEMENTARY   CHEMICAL   MICROSCOPY 


ring  with  thumb-screw  is  substituted  for  the  simple  ring.  This 
wire  ring  is  useful  as  a  support  when  moderately  long  heating 
must  be  practiced  or  when  evaporations  over  a  tiny  flame  at 
moderate  temperatures  are  required. 

For  the  production  of  higher  temperatures  than  are  possible 
with  the  flame  of  the  Bunsen  burner,  a  blowpipe  will  be  found 
convenient.  The  usual  form  employed  in  the  blowpipe  analysis 
provided  with  a  platinum  tip  should  be  chosen  and  if  in  addition 
it  can  be  fitted  with  a  hot  blast  attachment  its  usefulness  will 
thereby  be  greatly  increased. 


Fig.  74.     Type  of  Small  Blast  Lamp  for  Microchemical  Analysis.     (X  §.) 


Where  the  work  table  is  supplied  with  compressed  air  a  minia- 
ture blast  lamp  of  the  type  shown  in  Fig.  74  is  an  invaluable  aid 
in  fusions,  production  of  high  temperatures,  preparation  of  tiny 
blown  glass  apparatus,  etc.;  having  two  joints  it  can  be  quickly 
adjusted  to  almost  any  position  and  can  even  be  employed  to 
heat  material  directly  under  the  microscope,  although  such  oper- 
ations are  best  performed  by  means  of  an  electric  current  since 
the  heat  may  thus  be  far  better  localized. 

Heating  preparations  while  subjecting  them  to  observation 
through  the  microscope  may  be  accomplished  by  means  of  the 
electrically  heated  hot  stage  (see  page  224)  or  by  a  tiny  flame 


USEFUL   MICROSCOPE  ACCESSORIES 


129 


obtained  from  a  glass  or  quartz  tube  drawn  out  and  bent  up  and 
supported  by  the  substage  ring  of  the  instrument,  the  rotating 
stage  having  been  removed  to  avoid  injury  and  the  preparation 
supported  on  an  asbestos  plate  provided  with  a  small  central 
orifice  for  the  passage  of  light.  It  is  obvious  that  when  moder- 
ately high  powers  and  temperatures  are  to  be  employed,  the 
objectives  must  be  kept  cool  either  by  means  of  a  strong  blast  of 
cold  air  or  by  water  jackets.  To  meet  these  conditions  specially 
constructed  microscopes  are  obtainable;  a  typical  instrument 
of  this  sort  is  shown  in  Fig  7,  page  29. 

Small  Tongs.  —  As  a  substitute  for  crucible  tongs  for  holding 
platinum  foil,  cups,  etc.,  a  pair  of  compression  arterial  forceps, 
Fig.  75,  will  be  found  to  be  a  valuable 
addition  to  the  equipment.  Forceps 
of  this  sort  hold  thin  material  tena- 
ciously since  they  lock  firmly  in  place, 
and  thus  the  fingers  do  not  become 
cramped  during  prolonged  heat  treat- 
ments. 

Work  Tables.  —  The  type  of  work 
table  chosen  by  the  chemist  upon 
which  to  place  his  instruments  and 
apparatus  for  microchemical  inves- 
tigations will  depend  largely  upon 
his  individual  preferences  or  upon 
the  character  of  the  work  he  is  called 
upon  to  perform. 

In  general  a  table  provided  with  FlG  ?5<  Surgical  Compression 
an  indentation  or  CUt-OUt  portion  Forceps.  Convenient  for  Hold- 
along  one  edge  will  be  found  to  pos- 
sess many  advantages  over  a  simple 
straight  edged  table.  The  worker,  sitting  well  up  into  the 
cut-out,  secures  support  for  his  arms  and  is  enabled  to  sit  up 
straighter;  thus  he  is  subject  to  far  less  fatigue  during  long 
observations  and  manipulations.  Moreover,  in  the  greater  part 
of  microchemical  analyses  or  examinations  more  or  less  corrosive 
vapors  or  gases  are  apt  to  be  given  off  which  it  is  desirable  to  keep 


ing    Small    Platinum    Cups    or 
Pieces  of  Foil. 


130  ELEMENTARY  CHEMICAL  MICROSCOPY 

as  far  away  from  the  microscope  as  possible  and  yet  the  instru- 
ment must  be  readily  and  immediately  accessible  without  ma- 
terial change  of  position.  The  indented  table  offers  a  ready 
solution  of  this  problem  for  if  the  microscope  be  placed  to  one 
side  of  the  indentation  and  the  micro-burner  and  reagents  on  the 
opposite  side  the  worker  has  only  to  swing  to  the  left  or  right  as 
the  case  may  be  to  change  his  position  from  the  most  convenient 
one  for  manipulations  to  that  for  microscopic  observation.  Fig. 
76  shows  the  construction  and  arrangement  of  a  convenient  work 
table  for  microchemical  investigations. 

When  an  indented  table  is  provided  with  drawers  as  shown  in 
the  illustration,  care  must  be  taken  in  the  construction  to  see 
that  the  depth  of  those  nearest  the  cut-out  section  is  not  so  great 
as  to  hit  the  knees  of  the  worker  as  he  swings  from  one  side  to 
the  other. 

The  table  top  should  be  of  close  texture  and  finished  in  a  dull 
lusterless  black.  A  polished  or  shining  top  should  be  avoided, 
since  reflections  therefrom  are  always  annoying  and  very  tire- 
some to  the  eyes.  Glass,  porcelain  or  stone  tops  should  there- 
fore be  finished  with  dull  or  "ground"  surfaces,  never  polished. 
Coarse-grained  woods  should  be  avoided  because  of  the  difficulty 
of  keeping  them  clean;  for  this  reason  the  author  prefers  table 
tops  of  white  wood  or  poplar,  stained  with  aniline  black,  un- 
polished and  unvarnished,  and  merely  rubbed  down  smooth. 

To  guard  against  disfigurement  and  corrosion  of  the  table  top, 
manipulations  are  performed  upon  a  square  piece  of  plate  glass. 
A  convenient  size  will  be  found  to  be  from  twelve  to  eighteen 
inches  square. 

When  possible  the  work  table  should  be  piped  for  gas  and  com- 
pressed air  and  be  furnished  with  binding  posts  or  switch  for 
electric  current  (direct,  when  available).  Running  water  is 
unnecessary. 

The  arrangement  of  instruments,  apparatus  and  reagents 
upon  the  work  table  is  shown  in  the  cut  and  needs  no  further 
comment. 

A  stool  adjustable  in  height  and  provided  with  a  swivel  seat 
may  be  said  to  be  practically  indispensable.     If  the  stool  has  in 


USEFUL   MICROSCOPE   ACCESSORIES 


131 


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132  ELEMENTARY   CHEMICAL  MICROSCOPY 

addition  an  adjustable  back  the  added  comfort  thus  secured 
cannot  be  overestimated. 

Radiants  for  Microscopic  Illumination.  —  The  modern  micro- 
chemical  laboratory  employs  as  sources  of  artificial  light  for 
microscopic  illumination  the  electric  current  or  the  acetylene 
light.  Gas  light  illumination,  using  Welsbach  mantles,  made 
incandescent  by  coal  gas,  alcohol  or  gasoline  vapors,  have  al- 
ready become  radiants  of  the  past,  and  the  oil  lamp  is  now  so 
very  rarely  used  as  to  need  no  comment.  If  Welsbach  lights 
must  be  employed  owing  to  lack  of  electric  current  or  calcium 
carbide,  preference  should  be  given  to  lamps  of  the  inverted 
mantle  type. 

Cylinders  containing  compressed  acetylene  gas  are  now  so 
widely  distributed  and  the  gas  relatively  so  inexpensive  (exclud- 
ing the  first  cost  of  the  container)  that  few  investigators  will  care 
to  be  bothered  with  carbide  gas  generators.  A  piece  of  thin 
faintly  blue  glass  placed  between  the  acetylene  flame  and  the 
mirror  of  the  microscope  yields  light  approximately  equivalent 
to  daylight,  so  far  as  color  values  are  concerned.1 

The  development  of  dark-ground  and  of  vertical  illuminators 
and  their  applications  has  been  accompanied  by  a  corresponding 
improvement  in  electric  lamps.  These  now  fall  in  one  of  several 
groups:  carbon  arc  lamps,  Nernst  glower  lamps,  tungsten 
filament  incandescent  lamps  or  mercury  vapor  lamps. 

Ordinary  microscopic  work  rarely  requires  an  arc  lamp  draw- 
ing a  current  of  more  than  4  or  6  amperes,  but  for  ultramicro- 
scopic  investigations  an  arc  of  15  to  30  amperes  is  desirable  and 
in  many  instances  absolutely  essential.  Many  styles  of  con- 
struction are  found  on  the  market.  Several  typical  lamps  are 
here  illustrated.  Fig.  77  shows  the  4  ampere  hand-feed  arc 
lamp  of  the  Bausch  and  Lomb  Optical  Company;  Fig.  78  that 
of  the  Spencer  Lens  Company;  and  Fig.  79,  the  automatic  4  to 
5  ampere  lamp  as  manufactured  by  E.  Leitz.     In  Fig.  24  an 

1  Wright,  ArtiHcial  Daylight,  Amer.  J.  Sci.  (4)  27  (1909),  98.  Quite  re- 
cently the  Corning  Glass  Works  of  Corning,  N.Y.,  has  perfected  a  combi- 
nation of  glasses  such  that,  when  employed  with  large  tungsten  lamps,  true 
artificial  daylight  is  obtainable  as  shown  by  spectroscopic  tests. 


USEFUL   MICROSCOPE   ACCESSORIES 


133 


inexpensive  but  very  convenient  type  of  more  powerful  arc  lamp1 
is  shown  in  partial  section. 

Arc  lamps  for  microscopic  illumination  should  always  have 


Fig.  77.     Microscope  Lamp;  Bausch  &  Lomb.     Arc  Type. 

their  carbons  at  right  angles,  or  approximately  so.  Direct  cur- 
rent arcs  are  far  better  than  alternating  current.  The  horizontal 
carbon  should  be  the  positive  pole  and  the  carbons  should  be  soft 


Fig.  78.     Microscope  Lamp;  Spencer  Lens  Co.     Arc  Type. 

cored.  By  this  means  the  crater  is  maintained  at  a  fixed  point 
and  the  condensing  lenses  of  lamps  or  of  special  stands  will 
project  an  image  of  the  crater  upon  the  microscope  mirror  or 

1  Sold  by  Wm.  Gaertner  &  Co.,  Chicago,  111. 


134 


ELEMENTARY   CHEMICAL   MICROSCOPY 


into  the  vertical  illuminator  without  getting  seriously  out  of 
alignment  as  long  as  the  arc  is  burning.  Unless  a  considerable 
sum  of  money  is  invested  in  a  very  high  grade  automatic  lamp, 
it  will  be  found  better  to  use  hand  feed  arcs.  Cheap  automatic 
lamps  are  rarely  satisfactory  and  it  is  only  when  expensive 


Fig.  79.     Microscope  Lamp;  E.  Leitz.     Arc  Type.     Automatic. 

outfits  are  purchased  that  steady  uninterrupted  feeding  of  the 
carbons  takes  place,  yielding  an  arc  of  uniform  brilliancy  and 
non-flickering  crater.  Hand  feed  lamps  are  therefore  to  be  pre- 
ferred for  ordinary  work.  Satisfactory  results  can  only  be 
obtained  from  good  carbons.  These  should  be  moderately  soft 
and  of  uniform  composition. 

In  most  cases  the  interposition  of  a  cell  filled  with  water 
between  the  arc  lamp  and  preparation  is  essential  in  order  to 
prevent  damage  to  optical  apparatus  and  specimens  by  heat. 
Filling  the  cell  with  a  solution  of  alum  or  ferrous  sulphate  is  no 
better  than  pure  water  alone. 

Next  to  the  carbon  arc,  the  Nernst  lamp  is  most  satisfactory, 
so  far  as  light  intensity  and  convenience  of  mounting  are  con- 
cerned. Fig.  80  shows  a  Nernst  glower  galvanometer  lamp1 
which  serves  admirably  for  microscopic  work,  especially  for  ob- 
taining oblique  illumination  in  the  study  of  opaque  objects  and 

1  Made  by  the  Cambridge  Scientific  Instrument  Co.,  Cambridge,  England. 


USEFUL   MICROSCOPE  ACCESSORIES 


135 


as  radiant  for  vertical  illuminators.  For  use  in  this  way  the 
cross  wire  just  outside  the  projection  lens  is  removed  as  well  as 
the  cross  wire  diaphragm  sliding 
into  the  tube.  It  sometimes 
happens  that  owing  to  a  drop 
in  the  voltage  and  a  high  resist- 
ance of  the  "ballast"  in  the  lamp, 
the  heater  will  not  raise  the  glower 
to  the  necessary  temperature  to 
permit  the  passage  of  the  electric 
current.  In  such  an  event  care- 
fully unscrew  the  lamp  from  the 
tube  and  hold  a  lighted  match 
under  the  glower.  The  glower 
will  usually  become  incandescent 
and  the  lamp  can  be  screwed  back  Fig.  80.  Galvanometer  Lamp  of  the 
in  place.  Cambridge     Scientific     Instrument 

A  more  powerful  Nernst  lamp  erns     ype 

is  shown  in  Fig.  81.     This  lamp1  is  intended  primarily  for  use 
with  dark-ground  illuminators.     As  supplied  by  the  dealers  this 


Fig.  81.     Microscope  Lamp.     Nernst  Type. 

lamp  has  a  ground-glass  globe.  A  more  convenient  arrangement 
consists  in  substituting  for  the  globe,  a  tin  shield  as  illustrated. 
The  chief  difficulty  encountered  with  single  glower  Nernst 
lamps  is  the  fact  that  the  radiant  is  long  and  very  narrow  and  its 
image  projected  into  the  field  fails  to  give  uniform  illumination 
1  Sold  by  Bausch  &  Lomb  Optical  Company,  Rochester,  N.  Y. 


136  ELEMENTARY  CHEMICAL  MICROSCOPY 

unless  great  care  is  taken  in  adjusting  the  distance  of  radiant, 
condensing  lenses,  diaphragms,  etc.  Multiple  glower  lamps  are 
far  superior  in  this  respect.  Unfortunately  they  are  so  fragile 
and  require  such  care  in  handling  as  to  render  them  expensive 
and  therefore  impracticable  for  the  average  chemical  laboratory. 

To  obtain  a  uniformly  illuminated  held  with  single  glower 
Nernst  lamps  recourse  must  be  had  to  a  screen  of  ground-glass. 
This  causes  a  diffusion  and  softening  of  the  light,  but  greatly 
reduces  its  intensity,  the  loss  being  from  30  to  60  per  cent,  accord- 
ing to  the  thickness  and  nature  of  the  glass  and  the  character 
of  the  ground  surface. 

The  most  satisfactory  electric  lamps  for  general  purposes  now 
available  are  Mazda  projection  lamps  with  concentrated  fila- 
ments. These  General  Electric  Company  lamps  have  round 
bulbs  and  are  made  for  1 10  volt  circuits  in  100  watt  and  60  watt 
sizes.1  They  may  be  obtained  with  plain  glass  bulbs  or  with 
frosted  glass  having  a  circular  unfrosted  window.  On  the  Cor- 
nell University  lighting  circuit  the  100  watt  lamps  have  ap- 
proximately 65  and  the  60  watt  lamps  49  candle  power.  Em- 
ployed with  screen  and  suitable  condensing  lenses  these  lamps 
leave  little  to  be  desired  where  a  moderately  powerful  radiant  is 
required.  The  'tungsten  filament  will  stand  rougher  treatment 
than  Nernst  glowers  and  is  not  subject  to  burning  out  through 
short  circuit.  They  yield  excellent  results  in  illumination  by 
transmitted  light  in  the  usual  manner  by  means  of  the  micro- 
scope mirror  or  as  a  source  of  light  in  dark-ground  illumination 
or  with  vertical  illuminators,  but  for  oblique  reflected  light  in 
the  study  of  opaque  objects,  the  size  of  the  lamp  bulb  and  the 
position  of  the  tungsten  filament  renders  the  lamp  and  condens- 
ers somewhat  clumsy  and  apt  to  be  in  the  way.  Fig.  82  shows 
the  construction  of  these  lamps.  They  are  now  supplied  by  the 
Bausch  &  Lomb  Optical  Company  with  the  same  style  of  stand 
as  shown  in  Fig.  81.  Because  of  the  very  little  difference  in  the 
candle  powers  of  the  two  lamps,  it  will  be  found  that  in  general 
the  60  watt  lamp  is  more  convenient  on  account  of  its  smaller 

1  Since  the  above  was  written,  200,  400  and  500  watt  nitrogen  filled  concen- 
trated filament  tungsten  lamps  have  been  placed  upon  the  market. 


USEFUL  MICROSCOPE   ACCESSORIES 


137 


bulb  diameter.  To  avoid  eye  fatigue,  when  using  one  of  these 
powerful  tungsten  lamps,  it  should  be  screened  or  treated  with 
frosting  compound  and  graphite  or  aluminum,  as  suggested  on 
page  87.  Two  or  three  dippings 
will  be  required  to  produce  a 
coating  absolutely  opaque.  A 
window  is  then  made  by  washing 
off  a  circular  area  with  alcohol. 

Nosepieces.  Objective  Chang- 
ers. —  In  ordinary  microscopic 
investigations  frequent  changes 
from  one  objective  to  another  in 
order  to  obtain  increased  magni- 
fication are  usually  necessary.  To 
avoid  the  annoyance  and  loss  of 
time  required  to  unscrew  one  ob- 
jective and  reinsert  another,  va- 
rious devices  have  been  suggested. 
Those  almost  universally  em- 
ployed by  biologists  are  known 
as  revolving  nosepieces  and  are  shown  in  Figs.  83  and  84. 
The  illustrations  show  their  construction  and  operation  suffi- 
ciently well  to  need  little  comment.  The  nosepiece  is  attached 
to   the  body   tube  of   the  microscope.     It  may  accommodate 


Fig.  82.     Tungsten  Lamp  with  Con- 
centrated Filament. 


Fig.  83.     Revolving  Nosepiece  for 
Three  Objectives. 


Fig.  84.     Dust-proof  Revolving 
Nosepiece. 


two,  three  or  four  objectives  as  the  case  may  be.  The 
better  type  is  shown  in  Fig.  84.  It  is  circular  and  almost 
dust-proof,  while  in  the  type  shown  in  Fig.  83,  if  by  chance  the 
objectives  are  not  turned  under  the  shields  dust  falls  upon  the 
back  lens  combinations.  Owing  to  the  almost  impossibility  of 
constructing   these  nosepieces  so   that  each  objective  will  be 


138 


ELEMENTARY   CHEMICAL   MICROSCOPY 


properly  centered  when  turned  in  place,  many  investigators  pre- 
fer "objective  holders"  or  "changers"  instead  of  revolving  nose- 
pieces.  Three  forms  of  objective  changers  are  illustrated  in 
Figs.  85,  86  and  87.     In  the  case  of  those  of  the  form  of  Figs.  85 


Fig.  '  85.      Bausch  &  Lomb    Clutch  Fig.   86.      Leitz   Clutch  Objective 

Objective  Changer.  Changer. 

and  86  a  flanged  collar  is  attached  to  each  objective.  Pressing 
the  levers  together  opens  the  clutch,  and  permits  the  objective 
with  collar  attached  to  be  pushed  in  place.     Upon  releasing  the 

levers  the  objective  is  seated  and  securely 
held.  In  the  case  of  the  Zeiss  device, 
Fig.  87,  the  objective  is  screwed  into  the 
sliding  block  b  and  is  pushed  into  the 
slides  in  the  plate  a  which  is  attached 
to  the  end  of  the  body  tube  of  the  mi- 
croscope. The  screws  S,  S',  turned  by 
a  small  key,  permit  the  accurate  center- 
ing of  each  objective.  This  is  the  best 
type  of  device  when  centering  is  essen- 
tial, but  requires  a  special  box  for  holding 
the  objectives  to  which  the  blocks  b  have  been  attached.  With 
the  clutch  or  clamp  type  (Figs.  85  and  86)  the  ring  is  of  such 
diameter  as  to  permit  placing  the  objectives  in  their  usual  brass 
boxes. 

Sedimentation  Glasses.  —  The  apparatus  illustrated  in  Fig.  88, 
commonly  known  as  Spaeth's  sedimentation  glass  will  be  found 
a  most  useful  laboratory  device.  The  liquid  containing  the  sedi- 
ment to  be  examined  is  poured  into  the  glass  with  its  stopcock 


If^sss; 

_— JU 

|  J 

Oasrf 

§i?     —y 

uf> 

*"~-~i^r — - — tn 

f  r  ■ 

Js      ■"-""  j 

JiM 

' "  "'"•. ~,. .,J| 

Fig.  87.     Zeiss  Centering 
Objective  Changer. 


USEFUL   MICROSCOPE  ACCESSORIES 


139 


up  as  shown.  After  subsidence  has  taken  place  gentle  stirring 
will  dislodge  any  material  clinging  to  the  sides  of  the  vessel  and 
this  will  fall  to  the  bottom.  The  stopcock  is  now  turned  a 
quarter  turn  and  the  liquid  emptied  out.  The  stopcock  can  now 
be  removed  with  the  sediment  contained  in  the  conical  depres- 
sion and  with  but  very  little  of  the  supernatant  liquid.  The 
apparatus  is  especially  useful  in  cases  where  fractional  separations 
through  variable  rates  of  subsidence  can  be  practiced. 


,■;::,;« 


Fig.  88.     Spaeth  Sedimentation 
Glass. 


Fig.  89.    Polarization  Tube. 
Bates  Type. 


The  Bates  Polarization  Tube.  —  It  sometimes  happens  that 
an  approximate  determination  is  wanted  of  the  specilic  rotatory 
power  of  a  substance,  but  no  polarimeter  is  at  hand  although 
a  chemical  microscope  with  polarizer  and  analyzer  is  available. 
By  introducing  a  tube  of  a  solution  of  the  substance  to  be  studied 
into  the  tube  of  the  microscope,  we  can  convert  this  instrument 
into  a  polarimeter.  A  convenient  form  of  observation  tube  for 
this  purpose  is  the  Bates1  polarization  tube,  Fig.  89.  The  tube 
is  filled  with  a  solution  of  the  substance  and  placed  within  the 
1  Made  by  the  Bausch  &  Lomb  Optical  Company,  Rochester,  N.  V. 


140  ELEMENTARY   CHEMICAL   MICROSCOPY 

draw-tube  of  the  chemical  microscope,  thus  converting  the  instru- 
ment into  a  Mitscherlich  polarimeter  of  simplest  possible  con- 
struction. 

The  results  obtained  are  approximate  only,  since  the  gradu- 
ated circles  usually  attached  to  the  analyzer  (or  polarizer)  are  of 
such  small  circumference  that  the  readings  are  rarely  accurate 
to  even  a  degree;  moreover,  the  end  point  is  generally  far  from 
being  sharp.  It  is  therefore  evident  that  the  polarizing  micro- 
scope with  inserted  tube  is  not  to  be  regarded  as  a  substitute  for 
a  polarimeter,  but  as  a  device  useful  in  qualitative  analysis,  and 
offering  a  means  of  obtaining  rough  quantitative  results. 

To  employ  the  microscope  as  a  polarimeter,  proceed  as  follows. 
Remove  all  condensing  lenses  from  above  the  polarizer.  Remove 
the  objective  of  the  microscope.  Rack  the  body  tube  down  as 
far  as  it  will  go.  Insert  the  empty  tube  in  the  tube  of  the  instru- 
ment; cross  the  nicols  and  note  that  their  zero  points  are  cor- 
rectly placed.  Fill  the  tube  with  the  solution  to  be  examined 
and  illuminate  with  parallel  light.  Between  radiant  and  plane 
mirror  place  a  plano-convex  lens  to  assure  parallel  rays.  It  will 
also  generally  be  found  essential  to  employ  ray  filters  giving 
yellow,  approximately  monochromatic  light. 

It  is  even  better  to  incline  the  body  of  the  microscope  until 
the  tube  is  in  a  horizontal  position,  swing  the  mirror  to  one  side 
and  project  the  beam  of  parallel  light  upon  the  polarizer.  A 
dark  cloth  thrown  over  the  instrument  and  the  head  of  the  ob- 
server prevents  light  from  entering  between  the  polarizer  and  the 
tube  of  the  microscope  and  any  side  light  from  entering  the  eye. 

Upon  looking  into  the  microscope,  the  field  will  no  longer  be 
dark  gray  or  black.  Turn  the  analyzer  until  the  field  again 
acquires  its  maximum  darkness  and  read  the  scale.  The  amount 
of  displacement  to  the  right  or  left,  as  the  case  may  be,  is  the 
rotation  of  the  solution.  Dextrorotatory  substances  give  a 
smaller  angle  when  the  nicol  is  turned  to  the  right,  to  obtain 
maximum  darkness,  than  when  turned  toward  the  left;  while 
laevorotatory  substances  will  give  the  smaller  angle  when  the 
displacement  from  zero  is  to  the  left  than  when  to  the  right.  In 
all  cases  a  series  of  angle  measurements  should  be  made  and  the 


USEFUL   MICROSCOPE   ACCESSORIES  141 

average  taken.     It  is  obvious  that  in  this  series,  the  first  measure- 
ments must  include  rotation  of  the  analyzer  both  to  the  right  and 
to  the  left. 
The  specific  rotatory  power  of  a  substance  for  yellow  light, 

(a)D,  is  found  from  the  equation  («)D  =  — - — ,  where  a  is  the 

Ic 

angle  of  rotation  found,  c  the  number  of  grams  of  substance  in 
100  cubic  centimeters  of  solution  and  /  the  length  of  the  polariza- 
tion tube  employed  expressed  in  decimeters. 

Since  in  most  cases  the  specific  rotatory  power  of  a  substance 
is  known,  we  may  determine  the  per  cent  of  the  optically  active 
substance  by  dissolving  a  known  weight  of  the  material  contain- 
ing it  in  water,  making  the  volume  up  to  100  cubic  centimeters 
and  determining  the  angle  of  rotation  a  in  the  Bates  tube.  This 
tube  is  100  millimeters  long.  In  the  above  equation  all  the  mem- 
bers will  thus  be  known  but  c,  i.e.,  the  number  of  grams  of  the 
active  substance  present  in  the  mixture.  Solving  for  c  will  give 
the  result  sought. 

For  further  details  the  student  is  referred  to  the  standard 
works  on  the  polarimeter  and  saccharimeter. 


CHAPTER  VII. 
MICROMETRY  —  MICROMETRIC   MICROSCOPES. 

The  methods  commonly  employed  for  the  measurement  of 
minute  objects  by  means  of  the  microscope  may  be  conveniently 
grouped  under  six  different  heads. 

i.  Comparing  the  object  with  a  standard  scale  laid  in  juxta- 
position on  the  stage  of  the  microscope  within  the  field  of  vision. 

2.  Measuring  the  object  by  means  of  a  camera  lucida  and 
stage  micrometer. 

3.  Measuring  the  size  of  the  real  image  of  the  object  in  the 
microscope  and  dividing  by  the  magnification  of  the  instrument. 

4.  Measuring  the  object  by  means  of  an  ocular  micrometer, 
the  value  of  whose  scale  divisions  are  known. 

5.  Measuring  the  object  by  projecting  into  the  field,  by 
means  of  a  substage  condenser,  a  scale  of  known  value. 

6.  Measurements  obtained  by  the  graduated  head  of  the  fine 
adjustment. 

At  the  present  time  substantially  all  measurements  of  micro- 
scopic objects  are  recorded  in  microns  and  universally  desig- 
nated by  the  Greek  letter  n-  A  micron  is  one-thousandth  of  a 
millimeter.  In  the  case  of  submicroscopic  objects,  as,  for  exam- 
ple, the  exceedingly  minute  particles  demonstrated  by  the  ultra- 
microscope,  a  still  smaller  unit  becomes  necessary  in  order  to 
avoid  the  use  of  cumbersome  figures.  To  meet  this  need  the 
term  submicron  or  ultramicron  has  been  proposed  for  a  value 
equal  to  one-thousandth  of  a  micron,  the  designation  to  be  mx. 

All  micrometric  measurements  with  the  compound  microscope 
necessarily  partake  of  the  nature  of  close  approximations;  the 
more  skillful  and  experienced  the  investigator  the  more  nearly 
will  the  values  obtained  approach  the  true  dimensions  of  the 
object. 

142 


MICROMETRY  — MICROMETRIC   MICROSCOPES  143 

According  to  Rogers1  it  is  impossible  to  obtain  true  values 
with  certainty  closer  than  ±0.2  n,  this  value  being,  as  we  have 
already  seen,  the  practical  limit  of  the  resolving  power  of  the 
compound  microscope  (see  page  15).  But  when  a  series  of 
measurements  are  made  of  the  same  object  the  values  obtained 
will  usually  agree  among  themselves  by  less  than  0.2  n,  and  two 
different  experienced  microscopists  may  be  expected  to  obtain 
values  which  will  differ  by  less  than  this.  Ewell 2  believes  that 
microscopic  measurements  may  be  relied  upon  as  accurate  among 
themselves  within  less  than  0.1  n  or  even  under  exceptionally 
favorable  conditions  within  0.05  fx. 

The  degree  of  accuracy  obtained  will  obviously  be  largely  de- 
pendent upon  the  resolving  power  of  the  objective  employed. 

Micrometric  measurements  obtained  with  moderate  magnifi- 
cations are  much  more  accurate  as  a  rule  than  those  obtained 
with  high  powers. 

Method  1.  —  The  method  of  direct  comparison  of  object  and  scale 
is  generally  impracticable  and  seldom  available  where  ordinary 
microscopes  are  employed,  since  it  is  next  to  impossible  to  have 
object  and  scale  lie  in  exactly  the  same  plane  under  the  micro- 
scope. But  in  "micrometer"  or  "traversing"  microscopes  the 
principle  made  use  of  is  substantially  that  of  a  direct  comparison 
with  a  micrometer  scale. 

Since  the  chemist-investigator  is  not  infrequently  called  upon 
to  make  long  series  of  microscopic  measurements  of  objects  or 
to  measure  the  distance  between  lines  in  photographs  of  spectra, 
etc.,  types  of  these  special  micrometric  microscopes  are  shown  in 
Figs.  90,  91  and  92. 

For  the  comparison  of  lines  in  small  spectra,  scale  rulings, 
etc.,  the  traversing  microscope  shown  in  Fig.  90  3  will  be  found 
accurate  and  convenient.  This  instrument  consists  of  two 
microscopes  A  and  B,  mounted  in  fixed  positions  on  a  single 

1  Rogers,  W.  A.,  Proc.  Am.  Soc.  Micros.,  1883,  198. 

2  Ewell,  J.  Roy.  Micr.  Soc.,  1910,  537.     Nelson,  ibid.,  1910,  696. 

3  The  comparator  illustrated  in  Fig.  90  is  manufactured  by  Carl  Zeiss.  For 
methods  for  determining  the  corrections  to  be  applied  to  micrometer  microscopes, 
consult  Scienti6c  Paper  No.  215,  U.  S.  Bureau  Standards,  by  A.  VV.  Gray,  Microm- 
eter Microscopes  (1913). 


144 


ELEMENTARY   CHEMICAL  MICROSCOPY 


Fig.  90.    Zeiss  Traversing  Microscope  or  Comparator. 

heavy  base.  The  stage  S  slides  to  the  right  and  left  in  grooves; 
it  is  provided  with  two  sections  S1,  S2,  of  which  S1  may  be  moved 
independently  by  means  of  the  micrometer  screw  M1,  thus  per- 
mitting a  very  exact  adjustment  of  a  point  or  line  under  the 
cross-hairs  of  the  microscope  B.  At  the  opposite  end  of  the 
stage  a  second  micrometer  screw  M2  displaces  the  entire  stage. 
The  section  s2  carries  a  finely  graduated  scale  whose  rulings 
are  read  by  the  reading  microscope  A  of  fixed  focus.     Each 


MICROMETRY— MICROMETRIC   MICROSCOPES 


MS 


Fig.  91.     Beck  Micrometer  Microscope. 

microscope  is  provided  with  a  fixed  ocular  with  cross-hairs  mov- 
able by  micrometer  screws  attached  to  graduated  drums  D1,  D2. 
The  pitch  of  the  micrometer  screws  is  identical  in  each  instru- 
ment. One  complete  revolution  of  a  drum  is  an  aliquot  part 
of  one  division  on  the  scale  of  the  stage  S2.  The  object  to  be 
studied  is  placed  upon  section  S1  of  the  stage  and  clamped  in 
place,  the  stage  S  having  been  first  moved  by  hand  by  the  knob  K 
to  the  most  convenient  place  for  beginning  the  measurements. 


146 


ELEMENTARY   CHEMICAL  MICROSCOPY 


The  drums  D1,  D2,  are  set  at  zero;  the  point  or  line  on  the  object 
from  which  measurements  are  to  start  is  brought  exactly  under 
the  cross-hairs  of  B  by  means  of  M2;  the  exact  position  with 
respect  to  the  scale  is  determined  by  the  reading  microscope  A 


Fig.  92.     Reichert  Micrometer  Microscope. 


with  great  accuracy,  using  the  ocular  micrometer.  The  entire 
stage  is  next  displaced,  by  hand  or  by  the  screw  M2,  to  the  right 
or  left,  as  the  case  may  be,  until  the  second  point  is  reached  and 
the  scale  again  read. 

In  order  to  vary  the  magnification  of  the  observing  microscope 
B,  the  objective  is  mounted  so  as  to  slide  up  and  down  in  the 
body  tube.  A  double-ended  pointer  P  attached  to  the  objective 
mounting  moves  over  two  scales,  one  of  which  indicates  the  mag- 
nification, the  other  the  ocular  micrometer  value  of  the  drum  D1. 
Microscope  B  is  focused  by  means  of  the  pinion  F,  light  being 
thrown  by  the  mirror  O  through  the  preparation  to  be  examined. 


MICROMETRY  — MICROMETRIC    MICROSCOPES  147 

A  simpler  type  of  traversing  micrometer  microscope  is  shown 
in  Fig.  91. 

Micrometer  microscopes  of  the  type  shown  in  Fig.  92  are  es- 
pecially adapted  to  the  measurement  of  small  areas  (diameters  of 
circles,  etc.),  as,  for  example,  in  the  determination  of  the  hardness 
of  metals  by  the  Brinell  method  of  pressing  a  hardened  steel  ball, 
by  suitable  appliances,  upon  the  object  whose  hardness  is  sought. 
The  area  or  the  depth  of  the  indentation  resulting,  per  kilogram 
of  force  applied,  is  used  as  a  comparative  measure  of  hardness.1 

In  the  instrument  illustrated  in  Fig.  92  s  the  movement  of  the 
microscope  is  forward  and  back.  Focusing  is  accomplished  by 
the  pinion  F.  The  coarse  adjustment  of  the  cross-hairs  is  made 
by  means  of  A  and  the  fine  adjustment  upon  the  object  by  means 
of  a.  An  inclined  mirror  M  illuminates  the  micrometer  scale 
which  is  read  through  the  lens  R. 

Method  2.  —  Measurements  obtained  by  means  of  a  stage  microm- 
eter and  camera  lucida.  Lay  the  object  upon  the  stage  under  the 
microscope,  over  the  ocular  of  which  some  form  of  drawing  camera 
has  been  placed.  Adjust  the  illumination  even  more  carefully 
than  in  ordinary  drawing,  using  axial  light.  Focus  sharply,  and 
carefully  sketch  the  outline  of  the  object  upon  drawing  board 
or  notebook,  using  a  very  hard  and  sharp-pointed  pencil.  The 
object  is  now  removed  and  replaced  by  a  stage  micrometer, 
the  instrument  focused  and  the  graduations  of  the  scale  traced 
upon  the  paper,  either  across  the  outline  of  the  object  or  near 
by.  The  distance  from  the  camera  to  the  paper  must  be  identical 
in  each  case.  The  dimensions  of  the  object  may  thus  be  ascer- 
tained easily  by  comparison. 

This  method  of  indicating  the  size  of  different  objects  in  draw- 
ings of  microscopical  subjects  by  means  of  tracings  of  a  stage 
micrometer  is  always  preferable  to  a  tabulation  of  numerical 
dimensions,  since  the  indication  is  a  graphic  one  and  appeals  to 
the  eye  at  once.  Moreover,  it  enables  another  investigator  to 
ascertain  any  dimensions  indicated  in  the  drawings. 

1  For  a  critical  discussion  of  micrometric  methods  as  applied  to  hardness  deter- 
minations see  Devries,  Comparison  of  Five  Methods  used  to  Measure  Hardness. 
U.  S.  Bur.  Standards,  Tech.  Paper  n,  July,  1912. 

2  Made  by  C.  Reichert,  Vienna,  Austria. 


148  ELEMENTARY   CHEMICAL   MICROSCOPY 

Method  3. — Measurements  calculated  from  the  magnifying 
power  of  the  microscope.  This  method  is  the  least  satisfactory 
of  those  discussed  and  is  rarely  employed,  but  needs  considera- 
tion since  it  involves  a  determination  of  the  magnifying  powers  of 
the  microscope. 

"The  magnification  of  a  compound  microscope  is  the  ratio 
between  the  final  and  virtual  image  and  the  object  magnified" 
(Gage1).  Magnification  depends  upon  the  power  of  the  objec- 
tive, that  of  the  ocular  and  the  distance  between  objective  and 
ocular,  and  any  change  in  any  of  these  will  alter  the  magnifying 
power  of  the  instrument. 

Magnifications  are  always  recorded  for  a  distance  of  250  milli- 
meters, the  distance  of  most  distinct  reading  vision  of  the  normal 
human  eye. 

The  magnification  of  a  compound  microscope  is  most  easily 
ascertained  by  holding  a  piece  of  ground-glass,  tracing  paper  or 
tracing  cloth  at  a  distance  of  250  millimeters  from  the  stage, 
excluding  all  side  lights  with  a  screen  or  dark  cloth.  The  pro- 
jected rulings  of  a  sharply  focused  stage  micrometer  are  meas- 
ured with  a  pair  of  dividers  or  a  scale.  Dividing  the  size  of  this 
image  by  the  actual  size  of  rulings  gives  the  magnification  for 
standard  distance. 

Instead  of  employing  a  screen  as  above,  we  may  employ  a 
drawing  camera,  using  as  above  a  stage  micrometer  and  pro- 
jecting the  image  upon  notebook  or  drawing  board  placed  at 
the  standard  distance  of  250  millimeters,  measuring  from  the 
upper  or  outer  face  of  the  prism  of  the  camera. 

When  an  Abbe  camera  lucida  is  used  with  the  microscope  in  a 
vertical  position,  in  order  to  obtain  the  standard  distance  of 
magnification,  one  measures  from  the  outer  surface  of  the  Abbe 
prism  (this  being  the  limit  of  the  uppermost  reflected  ray)  to  the 
mirror  and  from  the  mirror  to  the  drawing  paper  as  indicated 
in  the  diagram  abc,  Fig.  52,  page  103.  To  obtain  this  distance 
it  is  necessary  to  raise  the  notebook  BB  above  the  plane  of  the 
table  top.  With  the  paper  in  this  position  the  magnified  image 
of  the  stage  micrometer  rulings  is  sketched  or  the  intervals  taken 
1  The  Microscope,  10th  Ed.,  1908,  p.  119,  Ithaca,  N.  Y. 


MICROMETRY  — MICROMETRIC   MICROSCOPES  149 

with  dividers;  their  distance  apart  is  then  determined  by  means  of 
a  millimeter  scale  and  the  value  obtained  divided  by  the  actual 
value  of  the  stage  scale  corresponding  to  the  enlarged  image.  The 
quotient  gives  the  magnification  of  the  microscope. 

It  is  evident  that  any  note-book  record  of  magnifying  power 
of  the  various  possible  combinations  of  oculars  and  objectives 
must  be  accompanied  by  a  record  of  the  tube-length  employed  in 
the  measurements.  For  this  reason  in  determinations  of  mag- 
nification it  is  best  to  use  the  tube-length  for  which  the  objectives 
and  oculars  have  been  corrected.1  It  is  also  evident  that  the 
paper  BB  and  the  reflecting  mirror  M  must  be  so  placed  that 
the  axis  cb  ba  is  normal  to  BB  and  to  the  optic  axis  of  the  mi- 
croscope. If  for  any  reason  the  drawing  paper  must  be  inclined 
and  is  not  level,  in  adjusting  the  mirror  to  obtain  an  axis  normal 
to  the  paper,  it  should  be  recalled  that  when  light  is  reflected, 
the  angle  between  the  incident  and  deflected  ray  is  equal  to 
twice  the  angle  of  inclination  of  the  mirror.  Hence,  in  order  that 
the  axial  rays  shall  fall  normal  to  the  drawing  surface,  the  mirror 
of  the  camera  must  be  set  at  45  degrees.  But  if  so  placed, 
only  about  one-half  the  field  of  the  microscope  can  be  sketched. 
In  order  to  increase  the  available  field,  the  mirror  must  be 
tipped  at  an  angle  less  than  45  degrees  with  the  horizontal. 
This,  however,  causes  distortion,  unless  the  drawing  surface 
is  inclined.  The  amount  of  inclination  is  in  accordance  with 
the  law  of  reflection  stated  above,  that  is,  that  the  drawing  paper 
must  form  an  angle  with  the  horizontal  twice  as  great  as  the 
angular  amount  the  mirror  is  depressed  below  45  degrees. 

Having  the  records  of  the  magnifying  power  of  the  various 
possible  optical  combinations,  in  order  to  obtain  the  dimensions 
of  an  object,  it  is  only  necessary  to  measure  the  image  obtained 
with  the  camera  lucida  under  identical  conditions  and  divide 
this  value  by  the  magnification. 

Method  4. — Measurements  obtained  by  means  of  micrometer 
oculars.  Micrometer  oculars  are  usually  of  one  or  two  types 
of  construction:    (a)  those  whose  scale  graduations  consist  of 

1  For  a  very  comprehensive  table  of  the  tube  lengths  for  which  objectives  are 
corrected  by  different  manufacturers,  see  Gage,  The  Microscope,  p.  18. 


150  ELEMENTARY   CHEMICAL   MICROSCOPY 

rulings  of  equidistant  spaces  with  no  provision  for  varying 
the  spaces  or  relative  position  of  cross  lines,  and  (b)  those  hav- 
ing a  scale  consisting  of  a  fixed  or  movable  scale  traversed  by 
cross-hairs  displaceable  by  micrometer  screws  attached  to  the 
graduated  drums  indicating  the  magnitude  of  displacement. 
Micrometers  of  the  latter  sort  are  generally  known  as  filar 
micrometers  and  comprise  the  most  accurate  as  well  as  most 
convenient  microscopic  measuring  devices  in  use.  Only  their 
high  cost  prevents  their  more  general  employment. 

Since  in  micrometer  oculars  the  graduated  scale  is  so  placed 
as  to  fall  in  the  same  plane  as  that  of  the  real  image  formed  by 
the  microscope,  the  number  of  scale  graduations  covered  by  the 
image  gives  a  value  for  the  size  of  the  image  only  and  not  for  the 
object.  It  is  therefore  necessary  in  all  cases1  to  first  ascertain 
the  true  value  of  the  eyepiece  scale  with  respect  to  each  objective 
used.     This  is  accomplished  by  means  of  a  stage  micrometer. 

Focus  the  eye  lens  of  the  ocular  so  that  the  graduations  of  the 
ocular  scale  become  clear  and  distinct.  Lay  the  stage  microm- 
eter upon  the  stage  and  move  it  until  the  center  of  the  rulings 
falls  in  the  optic  axis  of  the  microscope,  focus  carefully  and  ad- 
just the  micrometers  by  turning  ocular  or  stage  or  both  until 
the  rulings  in  one  scale  are  parallel  to  those  in  the  other.  Move 
the  stage  micrometer  until  a  line  becomes  coincident  with  a  line 
of  the  ocular  scale.  Count  the  number  of  divisions  of  the  ocular 
scale  included  between  one  or  more  divisions  of  the  stage  microm- 
eter. Divide  the  value  of  the  stage  scale  by  the  number  just 
obtained.  The  quotient  equals  the  true  value  of  one  ocular 
scale  division.  It  is  usually  the  case  that  conditions  obtain 
giving  an  appearance  shown  in  Fig.  93.  It  is  obvious  that  in 
such  an  event  it  is  necessary  to  estimate  with  the  eye  what 
fractional  part  of  a  division  to  add  to  the  whole  number  of 
divisions  of  the  ocular  scale  included  in  one  division  of  the  stage 
micrometer.  Such  an  estimation  or  guess  introduces  a  serious 
error  into  our  method.     Moreover,  the  image  of  an  object  to  be 

1  An  exception  to  this  statement  is  to  be  found  in  ocular  micrometers  with 
scales  so  ruled  by  the  manufacturer  as  to  yield  with  objectives  supplied  for  use 
with  them  a  definite  value. 


MICROMETRY— MICROMETRIC   MICROSCOPES  151 

measured  rarely  covers  exactly  a  whole  number  of  divisions  of 
the  ocular  micrometer  and  we  are  obliged  to  make  a  guess  as  to 


Fig.  93.     Micrometer  Scales  Improperly  Adjusted. 

what  fraction  of  a  part  to  add.     Thus  there  are  two  estimates 
necessary  and  any  measurements  recorded  must  necessarily  be 


Fig.  94.     Micrometer  Scales  Properly  Adjusted. 

mere  approximations.     The  second  of  these  errors  cannot  be 
eliminated  in  micrometer  oculars  with  fixed  scales  having  rulings 


152  ELEMENTARY  CHEMICAL  MICROSCOPY 

of  non-variable  magnitude,  but  the  determination  of  the  ocular 
micrometer  value  may  be  made  more  exact  by  eliminating 
fractions  as  shown  in  Fig.  94,1  where  it  is  evident  that  a  whole 
number  of  ocular  scale  divisions  are  included  in  a  whole  number 
of  divisions  of  the  stage  micrometer.  This  is  accomplished  by 
altering  the  ratio  between  the  images  of  the  two  scales  through  a 
change  in  the  position  of  the  draw-tube.  Start  with  the  draw- 
tube  pushed  in  as  far  as  it  will  go.  Pull  it  out  one  or  two  milli- 
meters at  a  time,  lining  up  and  focusing  the  stage  scale  each 
time,  until  one  or  more  spaces  of  the  stage  scale  are  covered  by  a 
whole  number  of  divisions  of  the  ocular  scale.  With  the  class 
of  objectives  commonly  employed  of  comparatively  low  powers, 
the  use  of  a  tube  length  slightly  different  from  that  for  which 
the  lenses  are  designed,  effects  their  resolving  power  so  little  as 
to  be  negligible.  In  order  that  the  conditions  may  be  duplicated 
under  which  the  ocular  micrometer  value  has  been  obtained,  it 
is  obvious  that  a  record  must  be  made  of  the  draw-tube  length 
employed;  the  notebook  entry  will,  therefore,  take  some  such 
form  as  this: 

16  millimeter  objective,  draw- tube  175;  1  division  ocular  scale  = 
0.01  millimeter  —  10  /jl. 

When  high  power  objectives  are  employed  the  rulings  of  the 
stage  micrometer  will  appear  as  very  thick  or  coarse  lines.  It 
then  becomes  essential  to  observe  special  precautions  in  the 
adjusting  of  the  ocular  and  stage  scales,  for  if  the  adjustment 
shown  in  Fig.  95  C  were  to  be  followed,  it  is  evident  that  an  error 
will  be  introduced  equal  to  at  least  half  the  thickness  of  the 
coarse  stage  rulings.  Either  the  ocular  micrometer  scale  lines 
must  be  placed  at  the  center  of  the  coarser  stage  lines,  as  shown 
in  A,  or  the  ocular  lines  may  be  placed  at  the  right  or  left  edges 
of  the  stage  lines,  but  always  all  of  them  on  the  same  sides  as  shown 

1  Figs.  93  and  94  were  drawn  by  means  of  a  camera  lucida  and  therefore  show 
exactly  the  conditions  met  with.  Each  division  on  the  stage  micrometer  (the 
lines  crossing  the  entire  field)  equals  o.i  mm.  With  a  pair  of  dividers  compare 
the  magnitude  of  a  space  in  Fig.  93  with  one  in  Fig.  94.  It  will  be  found  that 
lengthening  the  draw-tube  has  changed  the  ratio  between  images  of  stage  and 
ocular  scales. 


MICROMETRY— MICROMETRIC   MICROSCOPES 


!53 


in  Fig.  95  in  B.  The  value  of  the  ocular  micrometer  scale  must 
be  determined  for  each  objective  in  turn,  adjusting  the  draw- 
tube  in  every  case  so  as  to  avoid  estimating  fractions  of  a  scale 
division  and  in  each  case  the  record  must  be  kept  of  the  tube 
length  under  which  the  observations  were  made. 


i  n 

i 

B 

C 

1 

" 

i 

Correct 


Correct 


Incorrect 


Fig.  95.     Determining  the  Ocular  Micrometer  Ratio:  Heavy  Lines  =  Stage 
Micrometer,  Light  Lines  =  Ocular  Micrometer. 


In  the  ordinary  micrometer  ocular  it  is  often  somewhat  of  an 
eye  and  mental  strain  to  count  the  number  of  scale  divisions, 
especially  if  the  object  is  relatively  large.  To  facilitate  counting, 
Leitz  has  placed  upon  the  market  a  scale,  part  black,  part  light, 
in  which  the  divisions  are  sharply  differentiated  in  blocks  of 
ten,  both  horizontally  and  vertically.  This 
type  of  ruling  has  received  the  name  of  Step 
Micrometer,  and  is  far  less  fatiguing  to 
employ  than  the  older  simple  ruling.  Fig. 
96  shows  part  of  the  scale  of  a  step  microm- 
eter. Instead  of  being  ruled  in  tenths  and 
hundredths  of  a  millimeter  as  usual,  such  a 
value  is  used  by  Leitz  that  when  Leitz  ob-  Fig.  96.  Method  Em- 
jectives  are  employed  on  a  Leitz  microscope,      ployed  in  Ruling  the 

....  ,  Leitz  Step  Micrometer 

it  is  only  necessary  to  set  the  draw-tube  at      Ocular, 
the  point  indicated  for  that  particular  ob- 
jective.    The  ocular  micrometer  value  is  obtained  from  a  table, 
supplied  with  the  instrument.     Calibration  by  means  of  a  stage 
micrometer  is  therefore  unnecessary. 

For  measuring  bright  or  self-luminous  bodies,  such  as  the 
incandescent  filaments  of  lamps,  etc.,  the  Gebhardt  Contrast 
Micrometer,  Fig.  97,  made  by  Zeiss,  will  be  found  useful.  In 
place  of  line  rulings,  which  would  be  practically  invisible,  the 
scale  consists  of  a  row  of  tiny  black  squares  touching  at  their 


154 


ELEMENTARY   CHEMICAL   MICROSCOPY 


corners.     A  scale  of  this  type  will  stand  out  sharply,  no  matter 
how  bright  the  object  may  be. 

Filar  Micrometers.  - —  In  micrometry  with  oculars  having  fixed 
scales  there  is  always  the  probability  of  considerable  error,  as  we 

have  seen,  since  the  magnitude  of 
the  real  image  as  measured  by  the 
ocular  scale  usually  requires  a  guess 
as  to  just  how  much  of  the  scale  is 
included.  Very  minute  objects  even 
with  high  magnification  may  fail  to 
yield  real  images  of  sufficient  size  to 
even  fill  a  single  division  of  the  oc- 
ular scale.  To  meet  conditions  such 
as  these  filar  micrometers  are  em- 
ployed. In  instruments  of  this  kind, 
a  set  of  cross-hairs  are  made  to 
traverse  a  fixed  scale  by  means  of  a  screw  provided  with  microm- 
eter thread,  the  amount  of  the  movement  of  the  cross-hairs 
being  indicated  by  the  revolution  of  a  drum  attached  to  the  screw 
head.     A  typical  instrument  of  this  class  of  micrometer  oculars 


Fig.  97.  Zeiss  Contrast  Microm- 
eter Ocular  for  Measuring  Bright 
Bodies. 


Fig.  98.     Bausch  &  Lomb  Filar  Micrometer  Eyepiece. 


is  shown  in  Fig.  98.  The  scales  and  measuring  devices  of  instru- 
ments of  this  class  differ  in  different  instruments.  Figs.  99  and 
100  illustrate  two  of  these  forms.  The  Zeiss  filar  micrometer  has 
an  immovable  scale  with  the  zero  at  a  fixed  point  as  shown,  Fig. 
99;  across  this  scale  there  moves  simultaneously  the  set  of  cross- 


MICROMETRY— MICROMETRIC   MICROSCOPES 


33 


hairs  CC,  and  the  parallel  indicator  lines  C.  One  complete 
revolution  of  the  drum  is  equivalent  to  one  of  the  small  divisions 
of  the  fixed  scale.  Fig.  ioo  shows  a  Bausch  &  Lomb  microm- 
eter ocular  whose  fixed  scale  consists  of  a  black  comb  of  equi- 
distant teeth;  the  center  of  each  tooth  is  marked  by  a  sharp, 


Fig.  99.      Scale   of    the  Zeiss 
Filar  Micrometer. 


Fig.  ioo.     Scale  of  the  Bausch  &  Lomb 
Filar  Micrometer. 


short  line.  One  complete  revolution  of  the  drum  is  exactly 
equivalent  to  one  division  of  the  comb;  turning  the  drum 
causes  the  line  C  to  move  across  the  field.  The  zero  line  O  and 
the  horizontal  cross-hair  HH  are  fixed  with  respect  to  the  comb, 
but  the  entire  system  of  comb  and  zero  point  may  be  displaced 
at  will,  by  a  set  screw,  to  the  right  or  left,  for  convenience  in 
measuring. 

Before  filar  micrometers  may  be  used  for  micrometry  the  value 
of  one  division  of  the  ocular  scale  must  be  ascertained  by  means 
of  a  stage  micrometer  with  the  draw-tube  of  the  microscope  in  a 
recorded  position. 

When  using  micrometers  in  which  the  diameter  of  the  image 
of  the  object  is  measured  by  the  movement  of  a  micrometer 
screw,  a  number  of  observations  should  be  made,  always  moving 
the  cross-hairs  in  the  same  direction  to  eliminate  "back-lash." 

Method  5.  —  Projecting  a  scale  of  known  value  into  the  field  of 
view  by  means  of  substage  condensers.  This  ingenious  and  prac- 
tically universal  method  appears  to  have  first  been  suggested 
by  Goring  about  1820,  and  was  rediscovered  by  Pigott  in  1870, 


156  ELEMENTARY   CHEMICAL   MICROSCOPY 

and  employed  by  Sorby  in  refractive  index  determination  in 
1878.  Again  revived  by  A.  E.  Wright  in  1890.  Thoroughly 
tested  out  by  Ives  in  1903  and  independently  rediscovered  by 
Clendinnen  in  1910.1  And  yet  in  spite  of  the  many  times  this 
principle  of  employing  a  scale  of  variable  value  as  a  standard 
has  been  independently  discovered  and  its  desirable  features 
pointed  out,  it  is  almost  never  referred  to  in  manuals  devoted  to 
microscopy. 

By  means  of  the  mirror  and  the  Abbe  condenser,  it  is  possible 
to  project  into  the  plane  of  the  object  lying  upon  the  stage,  the 
image  of  a  scale  whose  value  has  been  ascertained.  Both  scale 
and  object  are  magnified  together  and  it  therefore  follows  that 
no  matter  what  may  be  the  combination  of  objective  and  ocular 
employed,  the  value  of  the  divisions  of  the  scale  image  will 
remain  unchanged,  provided  that  the  distance  of  the  scale  from 
the  condenser  is  not  altered.  Any  change  in  the  distance  of 
scale  from  mirror  and  condenser  will  be  accompanied  by  a  pro- 
portional change  in  the  size  of  the  divisions  of  the  scale  in  the 
image  projected  into  the  plane  of  the  object. 

In  micrometry,  by  means  of  ocular  micrometers,  we  are  re- 
stricted to  the  single  ocular,  containing  the  scale,  and  to  a  fixed 
draw-tube  length.  To  obtain  a  different  magnification,  one  is 
obliged  to  change  objectives.  This  means  that  a  new  ocular 
micrometer  value  must  be  employed  and  a  record  kept  for  every 
change  in  objective.  Moreover,  the  actual  sizes  of  the  divisions 
seen  in  the  eyepiece  micrometer  are  constant  and  cannot  be 
changed. 

In  micrometry,  by  means  of  a  scale  image  projected  by  the 
condenser,  we  have  merely  to  record  the  distance  of  the  scale 
from  the  microscope  in  determining  its  value  and  we  may  then 
adopt  any  possible  combination  of  objectives,  oculars  or  tube 
lengths,  without  change  of  value. 

The  scale  employed  may  conveniently  consist  of  a  glass  posi- 
tive obtained  by  photographing  any  suitable  scale  drawn  in 
ink  or  pencil,  either  as  a  linear  scale  or  ruled  in  squares,1  and 

1  See  Ives,  Journ.  Frank.  Inst.,  154,  73;  Clendinnen,  J.  Roy.  Micro.  Soc, 
1910,  368. 


MICROMETRY— MICROMETRIC   MICROSCOPES  1 57 

making  a  positive  from  the  negative  thus  obtained.  This  screen 
is  held  in  a  vertical  position  for  use  in  transverse  slots  cut  in  a 
long,  narrow  block  of  wood  which  is  notched  at  the  end  so  as  to 
permit  its  being  always  placed  in  exactly  the  same  position 
against  the  base  of  the  microscope;  the  glass  screen  carrying 
the  scale  is  illuminated  from  the  back  and  the  mirror  of  the  micro- 
scope turned  and  tipped  so  as  to  throw  an  image  of  the  scale 
into  the  condenser.  Raising  or  lowering  the  condenser  focuses 
the  scale  upon  the  object. 

The  value  of  the  image  of  the  scale  is  determined  by  means  of 
a  stage  micrometer  and  the  position  of  the  screen  in  the  block 
(i.e.,  its  distance  from  the  Abbe  condenser)  recorded.  The 
nearer  the  scale-screen  to  the  condenser  the  larger  the  image  of 
the  rulings  and  the  farther  the  scale  the  smaller  the  image.  To 
obtain  the  best  results  illuminate  the  screen  with  artificial  light 
placed  behind  ground  glass. 

Method  6.  —  Micrometry  by  means  of  the  fine  adjustment 
micrometer  screw.  Most  microscopes  are  provided  with  a  fine 
adjustment  so  constructed  with  micrometer  screw,  accurately 
ground  wedge  or  cone  as  to  permit  measurements  of  the  thick- 
ness of  objects  through  a  determination  of  the  amount  of  dis- 
placement necessary  to  focus  the  instrument  upon  the  lower 
and  the  upper  surface  of  the  object.  The  amount  of  displace- 
ment is  indicated  by  a  graduated  head  or  drum  attached  to  the 
fine  adjustment  moving  past  a  fixed  index. 

The  value  of  one  scale  division  of  the  drum  is  usually  marked 
by  the  maker  upon  the  instrument  or  indicated  upon  the  table 
of  magnifications  accompanying  the  microscope  when  purchased. 
If  this  value  is  unknown  it  may  be  ascertained  by  placing  an 
object  of  known  thickness  having  parallel  sides  upon  an  object 
slide,  clamping  as  tightly  as  possible  to  the  slide  with  the  stage 
clips  and  focusing  first  upon  the  slide,  then  upon  the  upper  sur- 
face of  the  object.  The  difference  in  the  fine  adjustment  drum 
readings  will  give  the  number  of  divisions  equivalent  to  the 
thickness  of  the  object.  The  thickness  of  the  object  used  may  be 
determined  by  placing  it  edgewise  on  the  stage  and  measuring 
its  thickness  by  any  one  of  the  micrometric  methods  given  above. 


158  ELEMENTARY   CHEMICAL   MICROSCOPY 

When  employing  the  fine  adjustment  for  micrometric  meas- 
urements, always  make  all  movements  in  focusing  in  the  same 
direction,  otherwise  a  serious  error  will  be  introduced  due  to 
back-lash. 

If  a  piece  of  an  object  slide  is  used  for  calibrating  the  fine 
adjustment,  it  must  be  remembered  that  we  cannot  focus  first 
upon  the  lower  surface  through  the  slide,  then  upon  the  upper 
surface,  to  obtain  its  thickness,  owing  to  the  displacement  of  image 
due  to  the  higher  refractive  index  of  the  glass  than  that  of  air. 
This  phenomenon  enables  us,  however,  to  determine  the  thick- 
ness of  transparent  objects  when  their  refractive  indices  are 
known  by  proceeding  as  described  on  page  200. 

Micrometric  measurements  by  means  of  the  fine  adjustment 
are  often  called  for  in  chemical  work,  as,  for  example,  to  ascer- 
tain the  depth  of  corrosion,  weathering,  pits,  streaks,  etc.,  in 
the  surfaces  of  many  different  sorts  of  materials,  or  in  approxi- 
mating depths  of  penetration,  or  in  measuring  in  transparent 
bodies  the  displacement  of  images  due  to  changes  in  refractive 
index.  This  displacement  enables  one  to  calculate  the  refractive 
index  of  the  object. 

Measurement  of  Areas.  —  The  methods  employed  for  the 
determination  of  the  areas  occupied  by  microscopical  objects  is 
discussed  in  Chapter  X,  page  216. 


CHAPTER  VIII. 

POLARIZED  LIGHT  — THE  SIMPLE  POLARIZING  MICRO- 
SCOPE—CRYSTALS UNDER  THE  MICROSCOPE. 

Few  chemists  realize  the  value  of  employing  polarized  light 
in  connection  with  the  microscopic  examination  of  material 
to  be  analyzed,  and  few  appear  to  appreciate  the  great  saving 
of  time,  labor  and  reagents  that  such  an  application  generally 
affords.  Even  a  cursory  examination  with  the  most  simple 
polarization  devices  is  not  to  be  ignored. 

In  the  microscopic  study  of  material  of  unknown  composition, 
the  first  step  of  the  chemist  should  be  to  subject  it  to  polarized 
light. 

But  in  order  that  the  polarization  microscope  may  be  employed 
intelligently  in  the  analysis  of  inorganic  and  organic  materials, 
it  is  essential  that  certain  fundamental  concepts  of  optics  and 
of  crystallography  be  recalled;  otherwise  the  phenomena  ob- 
served may  not  be  properly  interpreted. 

In  ordinary  light  the  ether  vibrations  are  in  all  possible 
azimuths  and  the  path  of  vibration  of  any  single  ether  particle 
is  constantly  changing.  Light  so  changed  that  the  ether  vibra- 
tions take  place  in  a  direction  parallel  to  a  single  plane  is  known 
as  polarized  light.  Polarized  light  may  be  either  white  light  or 
monochromatic  light. 

All  transparent  (and  translucent)  bodies  behave  with  respect 
to  light  waves  in  one  of  two  ways:  (i)  they  are  optically  homo- 
geneous and  therefore  have  no  effect  upon  a  beam  of  light  sent 
through  them,  no  matter  what  the  direction  may  be;  such  bodies 
are  called  isotropic  and  exhibit  but  a  single  index  of  refraction; 
ether  waves  proceeding  from  any  point  are  spherical;  (2)  they 
are  not  optically  homogeneous  but  transmit  light  waves  with 
different  velocities  in  different  directions;  in  this  case  they  are 
called  cBolotropic  or  anisotropic;   ether  waves  proceeding  from  a 

!59 


160  ELEMENTARY   CHEMICAL   MICROSCOPY 

point  are  ellipsoidal.  In  the  first  class  are  found  the  so-called 
amorphous  bodies  and  substances  crystallizing  in  the  isometric 
or  cubic  system,1  while  in  class  2,  we  find  substances  crystallizing 
in  the  hexagonal,  tetragonal,  orthorhombic,  monoclinic  and  tri- 
clinic  systems,  and  occasionally  bodies  normally  isotropic  but 
which  under  certain  stresses  and  strains  lose  their  homogeneity 
in  one  or  more  directions.  If  instead  of  employing  ordinary 
light  in  which  the  ether  vibrations  are  in  all  possible  azimuths 
and  where  the  paths  of  vibration  of  the  ether  particles  are 
constantly  changing,  we  illuminate  the  objects  with  plane 
polarized  light  in  which  the  ether  vibrations  are  parallel  to  a 
single  plane  it  becomes  much  easier  to  ascertain  whether  the 
transparent  object  is  isotropic  or  anisotropic. 

To  study  the  optical  behavior  of  tiny  crystals  or  transparent 
bodies,  use  is  made  of  the  polarizing  microscope.  For  ordinary 
chemical  investigation  the  polarizing  apparatus  may  be  quite 
simple,  but  in  crystallographic  and  petrological  studies  elaborate 
and  most  carefully  constructed  and  adjusted  instruments  are 
essential;  with  this  latter  type  of  instrument2  the  chemist  rarely 
has  anything  to  do. 

The  polarizing  apparatus  of  the  commonly  employed  chemi- 
cal microscopes  usually  consists  of  two  nicol  prisms,  one  placed 
below  the  stage,  the  other  above  the  microscope  objective. 

A  nicol  prism  consists  of  a  long  rhomb  of  calcite  cut  length- 
wise in  an  oblique  plane  forming  angles  of  90  degrees  with  the 
upper  and  lower  faces  of  the  rhombs  and  cemented  together  again 
with  Canada  balsam,  see  Fig.  101.  If  a  ray  of  light  R  enters 
such  a  prism  it  is  polarized,  being  resolved  into  two  component 
rays  vibrating  at  right  angles  to  each  other.  One  of  these  rays 
O,  known  as  the  ordinary  ray  is  deflected  slightly  more  than  the 

1  Certain  crystals  belonging  to  the  isometric  system  behave  in  a  similar  manner 
to  optically  active  chemical  compounds  in  solution,  in  that  they  possess  the  power 
of  rotating  the  plane  of  polarization  of  light  sent  through  them,  either  to  the  right 
or  to  the  left,  independently  of  the  direction  of  transmission.  Such  anomalous 
crystals,  although  isotropic,  may  be  said  to  be  doubly  refractive.  This  phenomenon 
is  termed  circular  polarization. 

2  For  a  very  comprehensive  discussion  of  the  Petrological  Microscope,  see  F.  E. 
Wright,  Pub.  No.  158  of  the  Carnegie  Institution  of  Washington,  The  Methods 
of  Petrographic-Microscopic  Research. 


CRYSTALS    UNDER   THE   MICROSCOPE 


161 


other  and  strikes  the  balsam  cement  at  such  an  angle  as  to  be 
totally  reflected;    the  other  ray  called  the    extraordinary  ray, 

passes  through  the  prism  and  emerges  com- 
pletely polarized.  In  the  diagram  at  S  is 
shown  a  cross  section  of  the  rhomb.  The 
direction  vb  through  a  shorter  diameter  of 
the  prism  rhomb  is  the  plane  or  direction 
of  vibration  of  the  nicol.  If,  after  emerging 
from  the  first  prism,  the  extraordinary  ray 
be  sent  into  a  second  nicol  so  placed  that 
its  plane  of  vibration  is  coincident  with  or 
parallel  to  the  direction  vb  of  the  first,  the 


Fig.  ioi.  Construction 
and  Path  of  Light  Rays 
in  a  Nicol's  Prism. 


Position  of  the  Prisms  with 
Nicols  Crossed. 


ray  emerges  parallel  to  its  entrance  direction  at  R.  In  this  e 
position  the  nicols  are  said  to  be  parallel.  But  if  the  second  > 
nicol  be  turned  through  90  degrees,  thus  taking  a  position  such 
that  its  plane  of  vibration  intersects  that  of  the  first  at  90 
degrees,  the  extraordinary  ray  will  behave  as  though  it  were  the 
ordinary  ray  and  is  completely  turned  aside.  No  light  emerges 
from  the  upper  nicol.  In  this  position  the  nicols  are  said  to 
be  crossed,  see  Fig.  102.  The  arrows  indicate  the  planes  of  vi- 
bration in  the  direction  of  the  short  diagonal.1 

1  In  the  newer  polarizing  microscopes,  the  prisms  often  do  not  have  a  rhombic 
cross  section  and  therefore  their  planes  of  vibration  do  not  fall  in  the  direction  of  a 
short  diagonal.  The  position  of  the  planes  of  vibration  must  then  be  ascertained 
experimentally;   see  Weinschenk,  Das  polarizations  Mikroskop. 


1 62  ELEMENTARY  CHEMICAL  MICROSCOPY 

The  lower  nicol  placed  below  the  object  is  called  the  polarizer; 
the  upper  nicol,  above  the  object,  the  analyzer,  since  it  serves 
to  examine  or  analyze  the  light  transmitted  by  the  object. 
For  the  best  results  a  nicol  prism  must  be  about  two  and  one- 
half  times  as  long  as  it  is  thick.  A  long  prism  for  the  analyzer 
is  cumbersome  and  undesirable,  therefore  a  calcite  prism  ce- 
mented with  some  resin  having  a  different  refractive  index  than 
Canada  balsam  is  generally  employed;  these  devices  are  known 
as  Thompson,  Glan,  Ahrens,  etc.,  prisms  after  the  men  inventing 
them.1 

Anisotropic  crystals  so  act  upon  plane  polarized  light  passing 
through  them  as  to  resolve  the  ether  vibrations  into  two  com- 
ponents polarized  at  right  angles,  the  planes  of  vibration  of 
which  are  not  coincident  with  the  plane  of  vibration  of  the 
analyzer. 

If  an  anisotropic  substance  be  placed  between  crossed  nicols 
and  slowly  turned  it  will  be  found  that  in  certain  positions  some 
light  emerges  from  the  upper  nicol  while  in  other  positions  no 
light  emerges. 

In  order  to  conveniently  study  the  effect  of  the  crystals  upon 
the  polarized  light  issuing  from  the  polarizer,  it  is  best  that  the 
polarizer  be  so  mounted  as  to  permit  rotation,  and  in  many 
cases  it  will  be  found  a  great  convenience  if  the  mount  is  pro- 
vided with  a  scale  graduated  to  indicate  the  degree  of  angular 
rotation.  The  analyzer  may  either  slide  in  and  out  of  the  body- 
tube  of  the  microscope  or  may  fit  above  and  over  the  eyepiece. 
The  latter  style  of  mounting  is  often  preferable  for  general  chem- 
ical laboratory  work.  Analyzers  screwing  into  the  body-tube 
just  above  the  objective  are  undesirable. 

For  convenience  the  polarizer  and  analyzer  should  be  so  mu- 
tually arranged  that  when  slipped  in  place,  the  position  for 
crossed  nicols  is  at  once  fixed  without  the  necessity  of  testing 
each  time  for  complete  extinction  of  light. 

In  the  chemical  microscope  illustrated  in  Fig.  3,  page  20,  the 
mounting  of  the  polarizing  nicol  is  provided  with  a  stud  and  the 

1  For  a  very  comprehensive  description  of  the  various  types  of  prisms,  see 
Johannsen,  Manual  of  Petrographic  Methods,  p.  158.     McGraw-Hill,  1914. 


CRYSTALS    UNDER   THE   MICROSCOPE  163 

substage  ring  into  which  the  polarizer  fits  has  a  notch  into 
which  this  stud  fits.  The  analyzer  mounting  is  notched  and 
the  draw-tube  of  the  microscope  has  a  tiny  projecting  pin  at  S/ 
over  which  the  notch  slips.  When  working  with  instruments  of 
this  type,  always  see  that  the  studs  or  pins  are  seated  as  deeply 
into  the  notches  as  they  will  go,  then  set  the  graduations  of  both 
polarizer  and  analyzer  at  zero;  this  will  give  crossed  nicols  and  a 
field  of  maximum  darkness. 

The  analyst  should  always  subject  his  instrument  to  a  search- 
ing examination  and  satisfy  himself  that  it  is  properly  con- 
structed and  that  any  measurements  obtained  will  be  accurate 
and  reliable.  The  most  important  points  to  be  ascertained  are: 
(1)  whether,  when  the  graduated  circles  of  polarizer  and  ana- 
lyzer are  each  set  at  zero,  the  nicols  are  exactly  crossed;  (2) 
whether  the  directions  of  the  cross-hairs  of  the  oculars  lie  90  de- 
grees apart  and  correspond  to  the  planes  of  vibration  of  the 
crossed  nicols;  and  (3)  whether  the  graduations  on  the  rotating 
circles  of  polarizer  and  analyzer  are  equivalent  and  correspond 
to  the  graduations  on  the  circumference  of  the  stage. 

1.  Testing  for  Properly  Crossed  Nicols.  —  Remove  the  analyzer 
and  objective.  Set  the  plane  mirror  so  as  to  yield  the  brightest 
possible  field,1  replace  the  analyzer  and  set  both  nicols  at  their 
zero  point.  Screen  the  stage  (i.e.,  the  open  space  between  the 
body  tube  and  stage)  and  cover  the  head  with  a  dark  cloth. 
Now  observe  carefully  whether  the  nicols  thus  set  are  in  their 
position  of  maximum  extinction.  This  is  done  by  turning  one 
of  the  prisms  the  least  amount  possible  and  noting  whether  the 
field  becomes  darker  or  lighter.  Make  a  number  of  observations, 
closing  the  eyes  for  a  few  seconds  each  time  just  before  looking 
into  the  microscope. 

2.  Testing  the  Cross-hairs.  —  Having  adjusted  the  polarizer 
and  analyzer  to  the  proper  position  of  crossed  nicols  as  ascer- 
tained above,  attach  a  low  power  objective,  insert  a  cross-haired 
eyepiece  and  place  upon  the  stage  previously  centered  a  prepa- 

1  High  grade  petrographic  and  crystallographic  microscopes  are  tested  for 
properly  crossed  nicols  by  pointing  them  directly  at  the  sun.  See  Wright,  F.  E., 
Petrographic  Methods,  1.  c,  p.  62. 


1 64  ELEMENTARY   CHEMICAL   MICROSCOPY 

ration  of  some  salt,  exhibiting  parallel  extinction  and  crystal- 
lizing in  long  prisms  with  straight  edges.1  Center  a  good  crystal 
and  turn  the  stage  until  the  crystal  extinguishes  —  i.e.,  attains  a 
maximum  darkness;  its  edges  in  this  position  should  be  exactly 
parallel  to  one  of  the  cross-hairs.  Turn  the  stage  through  90 
degrees;  the  edge  of  the  crystal  must  now  be  exactly  parallel 
with  the  other  cross-hair.  If  in  either  case  exact  parallelism 
has  not  been  obtained,  the  cross-hairs  of  the  ocular  do  not  corre- 
spond to  the  planes  of  vibration  of  the  nicol  prisms. 

Centering  the  Stage.  —  Before  it  is  possible  to  make  obser- 
vations relative  to  the  behavior  of  crystals  or  other  substances 
toward  polarized  light  or  to  measure  crystal  or  extinction  angles, 
it  is  essential  that  the  rotating  stage  of  the  microscope  be  accu- 
rately centered. 

Place  a  half  slide  upon  the  stage  of  the  microscope,  holding 
it  securely  in  place  with  a  stage  spring  clip.  Focus  with  a  1  inch 
or  32  millimeter  objective  upon  the  upper  surface  of  the  glass 
slide,  moving  it  about  until  a  tiny  defect  or  mark  is  found.  Move 
the  slide  with  the  fingers  until  this  mark  or  tiny  particle  is 
brought  directly  under  the  intersection  of  the  cross-hairs  of  the 
eyepiece.  Rotate  the  stage.  If  the  stage  is  centered  the  mark  or 
particle  will  remain  under  the  intersection  of  the  cross-hairs. 
If  not  centered,  the  particle  will  move  in  a  circle  whose  circum- 
ference passes  through  the  intersection  of  the  cross-hairs  but 
whose  center  is  off  to  one  side.  Slowly  rotate  the  stage  until 
the  mark  has  made  a  complete  revolution,  fixing  in  your  mind 
the  position  of  the  center  about  which  the  particle  has  rotated. 
Now  turn  the  stage  until  the  particle  or  mark  reaches  its  maxi- 
mum distance  from  the  intersection  of  the  cross-hairs  and  by 
means  of  the  stage  centering  screws  bring  the  particle  to  the 
center  about  which  it  has  rotated.  Move  the  slide  on  the  stage 
with  the  fingers  until  the  particle  or  mark  again  falls  directly 
under  the  cross-hairs.  Rotate  the  stage.  It  will  now  be  found 
that  the  stage  is  nearly  but  not  quite  centered.  Rotate  again, 
noting  as  before  the  path  of  the  mark  or  particle,  and  the  position 

1  For  this  purpose  allow  a  drop  of  a  saturated  solution  of  mercuric  chloride,  or 
of  ammonium  sulphate  to  crystallize  very  slowly  upon  an  object  slide. 


CRYSTALS   UNDER   THE   MICROSCOPE  165 

of  the  center  of  the  circle  through  which  the  particle  has  moved. 
Bring  the  particle  to  this  center  and  again  test  the  accuracy  of 
the  rotating  stage.  Absolutely  perfect  centering  throughout 
an  entire  rotation  of  360  degrees  is  seldom  possible  in  the  case 
of  medium-priced  instruments.  Providing  the  centering  is  good 
through  a  half  rotation  (180  degrees)  satisfactory  measurements 
may  be  obtained. 

Since  microscopes  are  commonly  provided  with  non-centering 
revolving  nosepieces.  centering  the  stage  for  one  of  the  three 
objectives  will  not  answer  for  the  other  two.  Each  time  one  ob- 
jective is  substituted  for  another  by  turning  the  nosepiece  it  is 
usually  necessary  to  recenter  the  stage.  A  very  convenient  de- 
vice for  approximate  centering  is  to  have  a  disk  diaphragm  just 
fitting  into  the  stage  opening,  the  orifice  of  the  diaphragm 
being  a  minute  pinhole.  To  center  the  stage  lay  the  diaphragm 
in  place,  focus  upon  the  pinhole  and  bring  the  point  of  light 
exactly  under  the  cross-hairs  by  means  of  the  stage  centering 
screws;  or  a  circle  of  drafting  ink,  the  exact  diameter  of  the  stage 
opening,  can  be  drawn  on  thin  ground-glass  or  tracing  cloth 
with  a  dot  at  the  center;  this  serves  a  purpose  similar  to  that 
of  the  diaphragm. 

3.  Testing  the  Graduated  Circles  upon  Polarizer  and  Analyzer. 
—  Although  the  zero  points  may  be  properly  set,  it  may  happen 
that  the  graduation  in  degrees  of  one  of  the  nicols  is  incorrect. 
Turn  one  nicol  a  few  degrees,  note  the  scale  reading,  then  turn 
the  other  until  extinction  results;  read  the  scale;  the  reading 
upon  each  circle  should  be  the  same  number  of  degrees. 

4.  Testing  the  Graduated  Circle  upon  the  Circumference  of 
the  Stage.  —  Place  at  the  center  of  the  stage  a  preparation  con- 
taining long  prisms  of  a  salt  exhibiting  parallel  extinction.  With 
the  nicols  crossed  at  zero,  select  a  good  crystal,  center  it  and 
bring  its  long  prism  edge  coincident  with  a  cross-hair.  Now 
turn  polarizer  and  analyzer  several  degrees,  each  being  rotated 
an  equal  distance  and  therefore  maintaining  the  relative  positions 
of  crossed  nicols.  Read  the  graduated  circle  on  the  analyzer, 
read  the  position  of  the  stage  and  rotate  the  stage  until  the  crys- 
tal extinguishes.     Read  the  stage  circle.     The  angular  rotational 


166  ELEMENTARY   CHEMICAL   MICROSCOPY 

displacement  should  be  the  same  number  of  degrees  as  that  of 
the  nicols.  In  like  manner  compare  a  number  of  different  seg- 
ments of  the  stage  graduations.  In  all  cases  several  observations 
should  be  made  at  each  position,  the  mean  of  all  the  readings 
being  taken. 

Polarization  without  a  Nicol  Prism.  —  When  employing  the 
hot  stage  microscope  it  is  sometimes  essential  to  obtain  polarized 
light,  yet  have  the  substage  kept  clear.  A  polarizer  of  the  nicol 
or  other  analogous  prism  type  is  obviously  impossible.  Recourse 
must  then  be  had  to  polarization  by  reflection.  A  variety  of 
devices  have  been  proposed,  one  of  these  is  illustrated  in  the 
microscope  shown  in  Fig.  7.  In  this  type  the  light  is  twice 
reflected  below  the  stage  with  the  result  that  the  object  is 
illuminated  by  transmitted  plane  polarized  light.  The  analyzer 
may  consist  of  any  convenient  sort  of  prism,  placed  either 
above  the  eyepiece  or  mounted  to  slide  in  and  out  of  the  body- 
tube.  The  best  results  are  obtained  from  reflections  from 
tourmaline  plates  but  Cheshire 1  has  shown  that  fair  results 
can  even  be  obtained  from  a  thin  plate  of  glass,  ground  on  one 
side,  and  blackened  upon  the  ground  surface.  Light  reflected 
from  such  a  plate  is  polarized;  the  maximum  polarization  is 
obtained  when  the  angle  of  the  incident  light  is  56^  degrees.  The 
plate  may  be  mounted  permanently  at  this  angle  and  arranged 
to  slip  into  the  substage  ring,  or  in  chemical  work  involving  heat- 
ing with  a  flame  supported  by  the  substage  the  plate  may  lie 
upon  the  work  table,  its  angle  of  inclination  being  obtained 
by  means  of  a  protractor  and  the  plate  held  in  place  by  means 
of  plasticine  for  a  temporary  mounting.  A  very  simple  ar- 
rangement of  the  Cheshire  plate  may  then  be  as  indicated  in  the 
diagram,  Fig.  103,  the  support  being  an  ordinary  object  slide, 
while  the  polarizing  plate  consists  of  a  half-slide,  ground  upon  its 
lower  surface  by  rubbing  upon  a  piece  of  glass  carrying  very 
fine  emery  and  turpentine.  After  cleaning  off  the  abrasive,  the 
ground  surface  is  blackened.  A  small  mass  of  plasticine  is  placed 
upon  the  slide  and  the  polarizing  plate  is  pressed  down  until 
the  proper  inclination  is  obtained  as  indicated  in  the  diagram. 

1  J.  Quekett  Micro.  Club,  8,  353. 


CRYSTALS    UNDER   THE   MICROSCOPE  167 

Thus  prepared,  this  polarizer  is  pushed  into  the  opening  in  the 
horseshoe  base  of  the  microscope  until  the  center  of  the  plate 
falls  in  the  optic  axis  of  the  microscope,  the  mirror  of  the  instru- 
ment having  been  removed  or  swung  aside.  Light  thrown  upon 
the  plate  will  be  polarized  and  reflected  in  the  line  of  the  optic 
axis  of  instrument. 


Fig.  103.     Obtaining  Polarized  Light  by  Reflection. 

To  further  assist  the  student  in  the  application  of  the  polarizing 
microscope,  the  following  brief  synopsis  is  given  to  refresh  his 
memory  relative  to  his  crystallographic  knowledge. 

Fundamental  Crystallographic  Concepts.  —  According  to  the 
viewpoint  of  the  crystallographer,  crystals  are  polyhedra, 
bounded  by  plane  surfaces,  whose  forms  are  dependent  upon 
physical  and  chemical  properties  and  governed  by  the  correla- 
tion of  certain  internal  forces  or  attractions  which  we  may  call 
a  definite  internal  grouping  or  arrangement  of  molecules  or 
atoms. 

It  must  be  remembered,  however,  that  the  chemist  in  recent 
years  has  discovered  a  number  of  substances,  appearing  when 
illuminated  with  ordinary  light  as  thick  syrupy  liquids,  yet 
which  yield  optically  most  of  the  phenomena  observed  in  solid 
crystalline  bodies.  To  this  interesting  class  of  compounds  the 
terms  liquid  crystals,  crystalline  liquids,  or  flowing  crystals  have 
been  given. 

It  appears  probable  that  only  chemical  elements  and  their 
definite  compounds  form  crystals. 

Crystals  may  form  when  a  solid  phase  separates  from  a  liquid. 


1 68  ELEMENTARY   CHEMICAL   MICROSCOPY 

The  liquid  may  be  either  a  solution  or  a  molten  mass.  Crystals 
may  also  form  from  vapors  on  cooling. 

The  bounding  polygons  of  a  crystal  are  called  faces,  all  of 
which  are  symmetrically  placed  with  reference  to  systems  of 
imaginary  lines  termed  axes. 

The  angles  formed  by  the  meeting  of  these  bounding  polygons 
are  called  interfacial  angles,  which  may  be  acute,  right  or  obtuse, 
and  are  never  reentrant. 

A  study  of  the  interfacial  angles  of  chemical  compounds  is  of 
the  utmost  importance,  since  these  angles  are  constant  for  a 
compound,  in  the  case  of  similar  faces,  no  matter  what  its 
origin. 

Crystals  are  classified  into  six  systems  according  to  their 
symmetry.  A  plane  of  symmetry  is  any  plane  which  passed 
through  a  crystal  will  divide  it  into  two  parts,  one-half  being 
the  mirror  image  of  the  other. 

Crystal  axes  are  usually  designated  by  the  letters,  a,  b,  c,  of 
which  the  c  axis  is  always  the  vertical,  a  the  forward  and  back, 
and  b  the  right  and  left. 

The  six  different  systems  (so-called),  to  which  crystal  forms 
may  be  referred,  differing  from  one  another  by  the  varying  of 
the  symmetry  of  the  crystals,  are  also  often,  but  less  correctly, 
defined  as  differing  by  variations  in  the  relation  of  the  axes. 
It  has  been  proved  by  Groth  that  there  can  be  only  four  kinds 
of  axes  of  symmetry — twofold  (binary),  threefold  (ternary), 
fourfold  (quartenary)  and  sixfold  (senary).  The  equivalent  faces 
become  coincident  through  revolutions  of  180  degrees,  120  de- 
grees, 90  degrees  and  60  degrees  respectively.  In  crystallog- 
raphy, by  symmetry  is  always  meant  symmetry  of  direction,  not 
of  actual  form  or  position.  It  follows,  therefore,  from  the  above 
facts,  that  the  crystal  angles  are  constant,  definite  and  character- 
istic for  each  crystal  form,  and  for  each  substance  thus  crys- 
tallizing, and  that  substances  may  often  be  identified  by  the 
measurement  of  their  crystal  angles. 

Slow  chemical  replacement  processes  sometimes  cause  more 
or  less  complete  changes  in  the  composition  of  a  substance  with- 
out affording  an  opportunity  for  an  accompanying  change  in 


CRYSTALS   UNDER   THE   MICROSCOPE  1 69 

crystal  form.  Such  replacement  forms  are  met  with  in  minerals 
and  are  known  as  pseudomorphs. 

When  a  crystalline  substance  is  found  with  its  own  crystal 
outlines  it  is  said  to  be  idiomorphic. 

When  a  crystal  has  opposite  ends  different,  due  to  dissimilar 
faces,  it  is  termed  hemimorphic. 

Two  crystals  may  unite  to  form  a  double  or  twin  crystal. 
Unions  in  threes  or  fours  are  less  frequent. 

Many  chemical  compounds  are  known,  however,  which  form 
more  than  one  kind  of  crystal.  Such  substances  are  said  to  be 
dimorphous,  trimor  phous  or  polymorphous,  according  to  the  num- 
ber of  observed  kinds  of  crystals  which  they  form. 

The  six  crystal  systems  are  as  follows,  each  comprising  the 
number  of  simple  generic  forms  indicated: 

o     »  Number  of  Simple 

System  Generfc  Forms 

i.  Isometric  Seven. 

2.  Tetragonal  Seven. 

3.  Hexagonal  Seven. 

4.  Orthorhombic  Four 

5.  Monoclinic  Four. 

6.  Triclinic  Four. 

A  crystal  is  said  to  be  holohedral  when  all  its  planes  are  present. 
When  one-half  the  planes  are  present  (in  accordance  with  an 
established  law)  the  crystal  is  hemihedral;  and  if  only  one- 
quarter  the  possible  planes,  the  crystal  is  called  tetartohedral. 

Crystal  aggregates  uniting  in  such  a  manner  as  to  yield  branch- 
ing, fern-like,  moss-like  or  tree-like  forms  are  called  dendrites, 
and  the  mass  is  termed  a  dendritic  mass.  If  the  aggregate  con- 
sists of  more  or  less  long  hair-like  twisted,  curved  or  bent  crys- 
tals, it  is  said  to  have  a  trichiten  structure,  and  the  individual 
hair-like  bodies  are  called  trichites.  But  when  the  tiny  long 
narrow  crystals  are  straight  and  resemble  needles,  the  crystals 
are  said  to  be  acicular.  Tiny  globular  masses  of  radiating, 
acicular  crystals  are  called  spheruUtes  or  sphcro-crystals.  When 
these  radiating  aggregates  consist  of  anisotropic  crystals  they 


170  ELEMENTARY   CHEMICAL   MICROSCOPY 

are  characterized  by  a  more  or  less  symmetrical  black  cross  if 
viewed  between  crossed  nicols. 

Very  rapid  crystallization  gives  rise  to  the  formation  of  crys- 
tals imperfectly  developed,  the  growth  generally  being  most 
rapid  in  the  direction  of  the  axes  or  of  the  boundaries  of  the 
facial  polygons.  The  bodies  resulting  are  called  skeleton  or 
skeletal  crystals. 

Under  like  conditions  of  formation,  crystalline  compounds 
always  separate  not  only  in  the  same  crystal  system,  but  will 
assume  each  time  the  same  geometrical  form;  this  character- 
istic form  is  called  the  habit  of  the  compound  and  upon  this 
property  microchemical  methods  of  analysis  are  based.  Pro- 
viding we  can  control  the  conditions  influencing  the  formation 
and  the  separation  of  a  crystalline  compound  upon  a  glass  object 
slide,  we  may  be  reasonably  certain  that  in  every  experiment 
tried  not  only  will  we  obtain  exactly  similar  crystals  but  also 
that  the  great  majority  of  the  crystals  will  always  lie  upon  the 
slide  in  a  similar  position.1 

Crystals  in  the  course  of  their  growth  invariably  occlude 
mother  liquor  and  furthermore  will  be  found  to  contain  in- 
clusions of  air  or  gases,  and  by  virtue  of  adsorption  or  solid 
solution  phenomena  will  contain  foreign  matter  which  may  be 
present.  Theoretically,  the  separation  of  an  absolutely  pure 
crystal  of  a  salt  consisting  of  a  single  solid  substance  alone  is  an 
impossibility  when  dealing  with  a  mixture. 

When  the  foreign  matter  present  is  such  that  the  adsorptive 
power  of  the  salt  for  it  is  great,  not  only  may  the  crystal  habit  be 
profoundly  changed  but  the  color  and  the  characteristic  proper- 
ties of  the  salt  may  also  be  altered.  It  is  possible  to  thus  obtain, 
by  the  means  of  vegetable  and  aniline  dyes,  colored  crystals 
from  colorless  inorganic  salts.2 

Fundamental  Facts  —  Optical  Crystallography.  —  In  addition 

1  E.  von  Fedorov  has  recently  compiled  an  elaborate  set  of  tables  in  the  Zeit. 
Kryst.  Min.,  50,  513,  whereby  it  is  possible  to  identify  a  compound  through  its 
crystallographic  habit  and  properties.  It  is  suggested  that  this  mode  of  analysis 
be  called  Crystallo-Chemical  Analysis. 

See  also  Orelkin  and  Pigulevski,  J.  Russ.  Phys.  Chem.  46,  227. 

2  See  Gaubcrt,  Recherches  recentes  sur  les  facies  des  cristaux,  Paris,  191 1. 


CRYSTALS   UNDER  THE   MICROSCOPE  171 

to  their  characteristic  morphology,  crystals  exhibit  certain 
physical  and  optical  properties  according  to  the  crystal  system 
to  which  they  are  referred.  Chief  among  these  optical  proper- 
ties made  use  of  by  the  chemist  is  the  behavior  of  the  crystals 
towards  polarized  light. 

Optically,  crystals  are  either  singly  refractive  (isotropic)  or 
doubly  refractive  (anisotropic).  If  isotropic,  they  will  show  no 
change  when  rotated  upon  the  stage  of  the  microscope  between 
crossed  nicols.  If  anisotropic,  they  will  appear  alternately  light 
and  dark  as  the  stage  is  turned. 

If,  therefore,  a  crystal  be  placed  upon  the  stage  of  a  polarizing 
microscope  near  the  center  of  the  field  between  crossed  nicols 
and  the  stage  turned,  the  crystal  will  behave  in  one  of  two  ways: 
1.  It  will  remain  dark  throughout  a  complete  rotation  of  the 
stage,  that  is,  there  is  no  change  in  its  appearance  in  the  dark 
field.  2.  As  the  stage  is  turned  the  crystal  will  alternately 
become  bright  or  colored,  and  alternately  disappear  or  become 
dark  (extinguish).  In  this  case  two  possibilities  arise.  Either 
the  crystal  disappears  (extinguishes)  when  its  long  edges  coin- 
cide with  or  are  parallel  to  the  cross-hairs,  and  is  brightest 
midway  between,  or  the  position  of  extinction  is  not  on  the  cross- 
hairs, but  lies  a  little  inclined  to  (is  oblique  to)  the  cross-hairs. 
In  the  former  case  we  speak  of  the  crystal  as  having  parallel 
extinction,  and  in  the  latter  as  having  oblique  extinction.  Crystals 
exhibiting  a  lozenge  or  equilateral  rhomb  outline  and  which 
extinguish  when  the  cross-hairs  bisect  the  acute  and  obtuse 
angles  of  the  lozenge  (a  variant  of  parallel  extinction)  are  some- 
times said  to  exhibit  symmetrical  extinction. 

Anisotropic  or  doubly  refracting  crystals  further  fall  into  two 
groups:  I.  Those  which  exhibit  no  double  refraction  in  one  di- 
rection through  the  crystal  —  uniaxial  crystals.  II.  Those  which 
exhibit  no  double  refraction  in  two  directions  —  biaxial  crystals. 
•  Those  directions  parallel  to  which  there  is  no  double  refraction 
have  been  designated  as  the  optic  axes.  The  directions  vary 
slightly  according  to  the  wave  length  of  light  but  for  all  practical 
purposes  may  be  considered  as  constant  for  white  light. 

Crystals  belonging  to  the  tetragonal  and  hexagonal  systems 


172  ELEMENTARY   CHEMICAL  MICROSCOPY 

are  uniaxial.  •  Those  of  the  orthorhombic,  monoclinic  and 
triclinic  systems  are  biaxial.  When  doubly  refracting  crystals 
lie  in  such  a  position  that  their  optic  axes  are  parallel  to  the 
optic  axis  of  the  polarizing  microscope,  the  nicols  being  crossed, 
the  crystals  remain  dark  when  the  stage  is  rotated;  in  other 
positions  the  crystals  will  appear  alternately  bright  and  dark. 
The  optic  axis  of  a  crystal  may  thus  be  found  experimentally.  If 
an  anisotropic  crystal  remains  dark  (on  turning  the  stage)  in  one 
position  only  it  is  uniaxial ;]  if  dark  in  two  positions  it  is  biaxial. 

To  obtain  a  clue  as  to  the  probable  system  of  a  substance 
yielding  polarizing  crystals,  find  the  position  of  extinction,  read 
the  stage  and  remove  the  analyzer.  Now  turn  the  stage  until 
the  centered  crystal  has  its  crystal  boundaries  or  crystal  cleav- 
age lines  lying  coincident  with  the  cross-hairs.  Read  the  stage 
again.  Try  a  number  of  crystals  in  turn.  If  the  angle  is  o 
degrees  or  90  degrees,  in  all  the  crystals,  the  system  is  either 
tetragonal,  hexagonal  or  orthorhombic,  i.e.,  the  crystals  exhibit 
parallel  extinction.  If  the  angle  is  not  o  degrees  or  90  degrees 
the  crystals  are  monoclinic  or  triclinic. 

The  tetragonal  or  hexagonal  systems  are  not  to  be  differen- 
tiated save  through  their  crystal  form  and  crystal  cleavage. 

Directions  of  Vibration,  or  Axes  or  Directions  of  Elasticity.  — 
In  all  the  doubly  refracting  crystals  there  are  certain  directions 
through  them  in  which  the  light  rays  advance  or  are  transmitted 
with  a  greater  velocity  than  in  other  directions. 

"The  directions  of  vibration  (found  always  to  be  at  right 
angles  to  each  other)  of  the  light  rays  which  advance  with  maxi- 
mum or  minimum  velocity  and  a  third  direction  at  right  angles 
to  the  plane  of  these  directions  (corresponding  to  some  ray  with 
an  intermediate  velocity)  are  called  Axes  of  Elasticity."  l 

In  the  orthorhombic  system  the  axes  of  elasticity  coincide  with 
the  crystallographic  axes. 

In  the  monoclinic,  one  axis  of  elasticity  coincides  with  the 
b-axis,  the  other  two  axes  of  elasticity  are  in  a  plane  of  symmetry 
at  right  angles  to  b,  but  are  coincident  with  neither  the  c-axis  nor 
the  a-axis. 

1  Luquer,  Minerals  in  Rock  Sections.     New  York,  1898. 


CRYSTALS   UNDER   THE   MICROSCOPE  173 

In  the  triclinic  system  no  axis  of  elasticity  is  parallel  with  a 
crystallographic  axis. 

For  the  relations  between  axes  of  elasticity  and  refractive 
index,  see  Chapter  IX,  page  194. 

Observations  with  Converging  Polarized  Light.  —  Strongly 
converging  polarized  light  offers  one  of  the  most  valuable  methods 
of  petrographic  microscopic  research,  but  it  possesses  only  a 
very  restricted  value  for  the  chemist  in  microchemical  quali- 
tative analysis.  Although  it  affords  a  means  of  differentiating 
between  crystal  systems  and  thus  yields  information  not  ob- 
tainable by  parallel  polarized  light,  useful  optical  phenomena 
with  converging  polarized  light  are  obtainable  only  when  the 
light  is  sent  through  crystals  in  the  direction  of  the  optic  axis 
in  the  case  of  uniaxial  crystals  or  in  a  direction  perpendicular 
to  the  plane  of  the  acute  bisectrix *  in  the  case  of  biaxial 
crystals. 

Tiny  uniaxial  crystals  will  occasionally  be  found  in  a  prepara-* 
tion  lying  in  such  a  position  as  to  be  available  for  study  with 
converging  polarized  light;  but  in  the  case  of  biaxial  crystals, 
sections  must  be  cut.  It  is  obvious  that  cutting  thin  sections 
of  tiny  individual  friable  crystals  of  the  vast  majority  of  the 
salts  met  with  in  analytical  work  is  impracticable  or  impossible. 
Moreover,  the  information  eventually  obtained  would  not  war- 
rant the  expenditure  of  time  and  patient  labor  required. 

Interference  Figures.  —  When  a  section  of  a  uniaxial  crystal 
cut  perpendicular  to  the  optic  axis  is  placed  upon  the  stage  of 
the  polarizing  microscope,  illuminated  with  strongly  converging 
polarized  light  and  the  observer  looks  into  the  microscope  with 
crossed  nicols,  but  with  no  eyepiece  in  place,  he  will  see  a  black 
cross  with  a  series  of  spectrum-colored  concentric  circles.2  This 
image  is  known  as  the  interference  figure. 

Biaxial  crystals  in  sections  normal  to  the  acute  bisectrix  yield 
(in  typical  cases)  curved  black  bands  or  an  asymmetric  black 

1  The  acute  bisectrix  is  a  line  bisecting  the  angle  formed  by  the  intersection  of 
the  two  optic  axes. 

2  Or  in  the  case  of  circular  polarization,  the  arms  of  the  cross  do  not  intersect 
but  leave  a  central  light  space. 


174  ELEMENTARY   CHEMICAL   MICROSCOPY 

cross  superimposed  upon  spectrum-colored  lemniscates  or  hyper- 
bolas.1 

In  order  to  observe  the  interference  figures  with  the  chemical 
microscope,  place  the  condensing  lenses  above  the  polarizing 
nicol,  center  the  crystal  or  crystal  section.  Use  a  |  or  |  inch 
or  4-millimeter  objective.  Focus  the  preparation  and  light  well. 
Remove  the  eyepiece,  place  the  analyzer  in  its  proper  position 
upon  the  top  of  the  microscope  tube,  cross  the  nicols  and  look 
into  the  instrument.  The  interference  figure  will  appear  as  a 
tiny  image  situated  far  below  the  eye.  Petrographic  and  crys- 
tallographic  microscopes  are  generally  provided  with  a  specially 
constructed  lens  which  slides  into  the  microscope  tube  above 
the  analyzer  and  below  the  eyepiece.  With  this  device  (Ber- 
trand  lens)  the  interference  figure  is  greatly  enlarged  and  it  is 
unnecessary  to  remove  the  ocular,  but  in  all  instruments  without 
this  special  device  and  where  the  analyzer  fits  above  the  ocular, 
the  ocular  must  be  removed  in  order  that  the  interference  figure 
shall  be  visible. 

Interference,  or  axial  figures  as  they  are  also  sometimes  called, 
must  not  be  confused  with  the  black  cross  observed  in  spheru- 
lites  and  starch  granules  placed  between  crossed  nicols. 

Interference  or  Polarization  Colors.  The  Selenite  Plate.  — 
As  stated  above,  when  light  enters  an  anisotropic  crystal  it  is 
polarized  or  resolved  into  two  rays  vibrating  at  right  angles  to 
each  other.  These  two  rays  are  propagated  at  different  veloci- 
ties, hence  one  component  is  slightly  retarded  and  upon  emerging 
from  the  crystal  one  ray  is  slightly  behind  the  other  in  rate  of 
vibration;  they  are,  therefore,  vibrating  in  a  different  phase. 
If  the  crystal  lies  between  crossed  nicols,  these  rays  upon  enter- 
ing the  analyzer  are  again  split,  and  owing  to  the  difference  of 
phase  the  waves  interfere  and  color  results.  Hence  the  crystal 
will  appear  more  or  less  colored.  The  brilliancy  of  color  will  de- 
pend upon  the  character  (strength)  of  double  refraction  and  the 

1  For  a  very  comprehensive  discussion  of  interference  figures  see  Weinschenk  — 
Das  Polarizations  Mikroskop,  or  Weinschenk-Clark,  1.  c,  Chapter  V.  See  also 
Moses,  The  Characters  of  Crystals,  N.  Y.,  1899.  Luquer,  Minerals  in  Rock  Sec- 
tions, N.  Y.,  1898.  F.  E.  Wright,  Petrographic  Methods,  Chapter  V,  1.  c.  Johann- 
sen,  Petrographic  Methods. 


CRYSTALS   UNDER   THE   MICROSCOPE  175 

thickness  of  the  crystal.  In  the  position  of  extinction  there  is 
of  course  no  color. 

If  the  value  of  the  double  refraction  is  known,  the  thickness 
of  the  crystal  may  be  calculated  and  vice  versa.1  Polarization 
colors  are  of  greater  value  in  petrological  investigations  than 
in  chemical  analysis.  Nevertheless,  the  analyst  should  never 
neglect  to  note  the  colors  and  their  intensities  when  examining 
preparations  between  crossed  nicols.  A  valuable  clue  as  to  the 
probable  nature  of  the  material  under  examination  may  often 
be  thus  obtained,  since  if  brilliant  polarization  colors  are  seen 
we  may  conclude  that  the  substance  has  a  high  double  refraction 
and  we  may  thus  eliminate  from  further  consideration  sub- 
stances whose  double  refraction  is  so  weak  as  to  render  brilliant 
interference  colors  impossible. 

It  is  often  difficult  to  determine,  between  crossed  nicols  alone, 
whether  or  not  a  substance  is  anisotropic  if  its  double  refraction 
is  very  weak,  and  only  the  faintest  tints  of  gray  are  produced. 
Recourse  is  then  had  to  a  selenite  test  plate  cut  of  such  a  thickness 
and  orientation  that  when  placed  between  the  nicols  with  its 
direction  of  vibration  at  45  degrees  to  the  planes  of  vibration 
of  the  nicols  a  purple-red  interference  color  is  obtained.  This 
particular  shade,  known  as  red  of  the  first  order,  is  the  most  useful 
of  test  plate  interference  colors.  When  such  a  test  plate  is 
placed  either  above  or  below  the  very  weakly  polarizing  prepa- 
ration being  studied  the  change  of  phase  in  the  transmitted 
light  waves  is  such  as  to  produce  a  contrasting  color.  The 
entire  field  is  colored  red;  the  polarizing  materials  or  crystals  will 
therefore  appear  differently  colored,  according  to  their  thickness, 
upon  a  red  background.  Double  refraction  so  weak  as  to  pass 
unnoticed  will  thus  be  readily  recognized. 

The  selenite  is  also  most  useful  in  the  determination  of  extinc- 
tion angles  (q.v.),  in  ascertaining  the  optical  sign  +  or  —  of 
biaxial  crystals,  and  in  measuring  the  thickness  of  thin  polarizing 
rock  and  crystal  sections. 

1  For  a  full  and  comprehensive  discussion  of  interference  colors  and  their  appli- 
cation in  microscopy  the  student  is  referred  to  Weinschenk-Clark,  Petrographic 
Methods,  pp.  73-87,  or  Johannsen,  Petrographic  Methods. 


■37 


S 


176  ELEMENTARY   CHEMICAL   MICROSCOPY 

One  of  the  best  examples  of  the  every-day  practical  applica- 
tion of  the  polarizing  microscope  and  selenite  plate  by  chemists 
is  in  the  differentiation  of  pure  fresh  butter  from  very  old,  or 
process  butter  or  oleomargarine.  The  fat  of  fresh,  unmelted 
butter  thus  examined  yields  a  uniform  red  field.  Process 
butter,  melted  butter  and  oleomargarine  on  the  other  hand 
yield  a  field  mottled  in  many  colors. 

For  use  with  the  chemical  microscope  the  selenites  are 
usually  obtained   as  disks  with  two    black  dots    at    opposite 

ends  of  a  diameter,  Fig.  104. 
These  dots  locate  the  direction 
of  vibration  of  the  test  plate  as 
^  shown  in  the  figure  by  the  dotted 
arrow.  These  selenite  disks  are 
employed  as  follows:  After  cen- 
tering and  focusing  the  prepara- 
tion, the  selenite  disk  is  laid  upon 
the  eye-lens  of  the  ocular  in  such 
a  position  that  its  direction  of 
vibration    bisects    the   angles   of 

Fig.  104.     Selenite  Disk.    The  Arrow     ,  ,     .  , 

Indicates  the  Direction  of  Vibration.    the   CrOSS-hairs,    as   shown   in   the 

diagram.  Petrographic  micro- 
scopes are  generally  supplied  with  test  plates  mounted  in  a 
metallic  carrier  arranged  to  slide  into  the  tube  of  the  micro- 
scope in  a  slot  provided  for  this  purpose.  The  direction  of  the 
vibration  is  in  this  case  indicated  upon  the  mount  by  an  arrow. 

Absorption.  Pleochroism.  —  Many  compounds  have  the  power 
of  absorbing  part  of  the  light  rays  vibrating  in  certain  planes 
and  therefore  if  viewed  through  the  polarizing  microscope  with 
the  analyzer  removed  will  exhibit  a  change  of  light  intensity, 
in  certain  positions.  This  property  of  crystals  known  as  ab- 
sorption should  not  be  confused  with  a  change  of  color. 

All  anisotropic  substances  to  a  greater  or  lesser  extent  remove 
the  rays  of  certain  colors  in  certain  planes  from  white  light  sent 
through  them.  This  property  when  sufficiently  pronounced 
to  be  observable  with  the  normal  human  eye  is  termed  pleo- 
chroism.    Substances   are    tested    for   pleochroism    by   placing 


C 


CRYSTALS   UNDER  THE  MICROSCOPE  177 

them  upon  the  stage  of  a  polarizing  microscope,  removing  the 
analyzing  nicol  and  rotating  the  polarizer.  If  the  substance 
under  examination  is  pleochroic,  it  will  change  in  color  with  the 
rotation  of  the  prism.  In  the  event  of  the  polarizer  being  fixed 
and  incapable  of  rotation,  rotate  the  stage.  Always  carefully 
shade  the  preparation  with  the  hand  in  order  to  prevent  as  much 
as  possible  confusing  reflections. 

If  the  phenomena  observed  involve  a  two-color  change  the 
crystals  are  said  to  be  dichroic;  if  a  three-color  change  trichroic. 
Uniaxial  crystals  can  exhibit  only  a  two-color  change;  biaxial 
crystals  may  be  trichroic. 

Isotropic  crystals  possessing  a  high  adsorption  power  for  cer- 
tain coloring  matters  may  become  in  the  process  of  their  growth 
highly  colored.  These  crystals,  although  still  retaining  their 
isometric  habit  are  often  highly  pleochroic. 

Practical  application  may  be  made  of  the  phenomenon  of 
pleochroism  in  differentiating  between  different  textile  fibers  and 
different  paper  fibers  stained  with  certain  aniline  dyes.  Some 
species  of  fiber  exhibit  strong  pleochroism  and  others  weak. 

The  Measurement  of  Crystal  Angles  and  Extinction  Angles.  — 
Since  the  interfacial  angles  of  crystals  of  chemical  compounds 
are  always  constant  for  similar  faces  no  matter  how  the  compound 
may  have  been  prepared,  it  is  obvious  that  angle  measurements 
may  often  prove  of  the  greatest  value  in  the  identification  or 
differentiation  of  compounds  or  of  crystal  systems.  When 
crystals  are  of  sufficient  size  to  be  handled  determinations  of  the 
values  of  angles  by  means  of  some  form  of  goniometer  are  fraught 
with  no  great  difficulties,  but  when  the  crystals  are  microscopic 
and  cannot  satisfactorily  be  orientated,  the  problem  becomes 
exceedingly  difficult. 

Fortunately,  the  chemist  is  rarely  if  ever  called  upon  to  make 
very  accurate  angle  measurements;  rapid  approximate  readings 
are  usually  sufficient  for  analytical  work.  Moreover,  so-calh-d 
chemical  microscopes  are  incapable  of  yielding  angular  measure- 
ments of  the  degree  of  accuracy  required  in  crystallographic 
investigations. 

Great  accuracy  on  the  part  of  the  analyst  is  seldom  essential, 


178  ELEMENTARY    CHEMICAL   MICROSCOPY 

since  his  object  is  merely  to  ascertain  whether  the  crystal  under 
examination  is,  or  is  not,  a  certain  compound.  In  simple  inor- 
ganic analyses  angle  measurements  are  rarely  resorted  to,  but  in 
the  examination  of  organic  compounds  and  in  the  case  of  mix- 
tures of  inorganic  and  organic  substances,  the  measurement  of 
angles  may  often  prove  a  most  rapid  means  of  differentiation. 

Only  thin,  well-formed  crystal  plates  with  practically  perfect 
edges  should  be  selected  for  measurement.  Avoid  high  magni- 
fications. The  rotating  stage  having  been  previously  centered, 
the  preparation  is  moved  with  the  fingers  until  the  selected 
crystal  is  brought  under  the  cross-hairs  of  the  eyepiece.  One  of 
the  bounding  edges  of  the  angle  sought  is  made  exactly  parallel 
to  and  almost  in  coincidence  with  one  of  the  cross-hairs;  the  posi- 
tion of  the  graduated  circle  of  the  stage  is  noted  and  the  stage  is 
rotated  until  the  other  bounding  edge  of  the  angle  becomes  par- 
allel with  the  same  cross-hair.  The  graduated  stage  circle  is 
again  read.  The  difference  between  the  two  readings  is  the 
angle  sought. 

If  it  is  known  that  the  cross-hairs  in  the  eyepiece  are  exactly 
at  right  angles,  a  slightly  quicker  method  consists  in  measuring 
the  complement  of  the  angle  and  deducting  it  from  90  degrees. 
Or,  if  the  angle  be  obtuse,  measure  the  amount  that  is  greater 
than  90  degrees.  This  method  does  not  necessitate  as  careful 
centering  of  the  stage,  and  can,  therefore,  be  used  with  high 
powers  with  sufficient  accuracy  for  analytical  work.  It  is  essen- 
tial in  all  measurements  of  crystal  angles  that  the  instrument 
be  most  carefully  focused  upon  an  edge,  and  that  care  be  taken  to 
avoid  error  due  to  the  projection  of  an  image  of  another  edge 
through  the  crystal.  In  the  case  of  very  transparent  crystals 
it  is  sometimes  difficult  to  tell  which  is  the  proper  fine  (edge)  to 
employ,  unless  the  crystal  is  thin. 

For  the  measurement  of  solid  angles  where  several  planes 
meet,  the  crystals  must  be  of  sufficient  size  to  permit  their  being 
turned  first  in  one  position,  then  in  another.  Cementing  to  the 
point  of  a  needle  (method  of  Kley1),  imbedding  the  head  of  the 
needle  in  a  cork  and  cementing  the  cork  to  a  glass  slip  will  permit 

1  Kley,  Rec.  trav.  chim.  Pays-Bas,  19  (1900),  13. 


CRYSTALS    UNDER     THE    MICROSCOPE 


179 


of  the  crystals  being  sufficiently  easily  orientated  to  yield  fairly 
accurate  measurements. 

Or,  we  may  employ  the  glass  hemisphere  (see  page  117),  or 
the  orientating  apparatus  of  Klein  (page  117). 

Microscopes  having  fixed  stages  require  the  employment  of  a 
goniometer  eyepiece,  consisting  essentially  of  a  cross-hair  system 
rotating  in  conjunction  with  a  graduated  circle.  With  this 
device  the  centered  crystal  remains  in  a  fixed  position  and  the 
ocular  cross-hairs  are  rotated  in  such  a  manner  that  one  of  them 
is  first  made  parallel  to  one  boundary  edge,  and  then  to  the  other 
edge  of  the  angle  sought. 

Extinction  Angles.1  —  The  extinction  angle  of  a  crystal  may 
be  defined  as  "  the  angle  between  an  axis  or  direction  of  elasticity 
and  some  known  crystallographic  direction."  The  crystallo- 
graphic  direction  usually  adopted  by  chemists,  where  the  extinc- 
tion angle  is  employed  as  one  of  a  series  of  identity  tests,  is  the 
longest  edge  of  the  crystal  or  in  the  case  of 
rhomb-shaped  crystals  the  line  bisecting  the 
acute  angles. 

In  the  case  of  crystals  exhibiting  parallel  ex- 
tinction the  extinction  angle  may  be  consid- 
ered as  being  o  degrees.  Crystals  exhibiting 
oblique  extinction,  i.e.,  those  of  the  monoclinic 
and  triclinic  systems  yield  two  extinction 
angles;  but  it  is  customary  to  record  as  the 
extinction  angle  the  smallest  angle  obtained 
between  the  length  of  the  crystal  (cleavage 
lines  or  edges  being  used),  and  the  nearest  axis  of  elasticity. 
In  Fig.  105  the  extinction  angles  may  be  considered  as  the 
angles  6. 

If  the  analyst  is  sufficiently  well-trained  in  crystallography  to 
be  able  to  locate  the  c-axis  he  may  record  as  the  extinction  angle 
the  angle  formed  between  the  c-axis  and  the  nearest  axis  of 
elasticity.  This  value  is  that  most  often  taken  by  crystallog- 
raphers  as  the  characteristic  extinction  angle. 


.*■ 


\^ 


r^f 


Fig.  105.     Extinction 
Angles,  0,  6. 


1  See  Wright,  Measurement  of  Extinction  Angles;  Am.  J.  Sci.  (4),  26  (1908), 


349- 


l8o  ELEMENTARY    CHEMICAL   MICROSCOPY 

Since  in  feebly  polarizing  crystals  the  exact  point  of  extinction 
is  not  easily  determined,  a  measurement  of  the  angle  is  difficult 
and  annoying  unless  a  selenite  test  plate  is  employed  (see  page 
175).  When  employing  a  selenite  proceed  as  follows:  Place 
the  test  plate,  red  of  the  first  order,  so  that  the  plane  of  its 
direction  of  vibration  bisects  the  opposite  quadrants  of  the  cross- 
hairs of  the  ocular.  With  the  nicols  crossed  bring  a  typical  thin 
crystal  so  that  its  long  edge  (or  its  c-axis)  lies  parallel  to  a  cross- 
hair. A  red  field  is  seen  with  the  crystal  of  some  contrasting 
color.  Read  the  graduated  stage  circle.  Now  slowly  rotate 
the  stage  until  the  crystal  acquires  exactly  the  same  color  as  the 
field;  the  plane  of  vibration  of  the  selenite  and  that  of  the 
crystal  are  now  coincident.  Read  the  stage  again.  The  read- 
ing will  give  an  extinction  angle.  Next  ascertain  whether  it  is 
the  smaller  of  the  two  possible  angles  for  this  position  of  the 
crystal.  Make  similar  measurements  upon  a  number  of  other 
crystals. 

Never  depend  upon  observations  made  upon  a  single  individ- 
ual. Check  the  readings  by  again  making  the  crystal  parallel 
to  the  cross-hairs  and  turning  the  polarizer  or  analyzer  until 
the  colors  of  field  and  crystal  are  identical;  read  the  gradua- 
tions on  the  nicol  mounting;  the  angles  observed  should  be 
identical. 


CHARACTERISTICS    OF    THE    SIX    CRYSTAL    SYSTEMS. 

SUMMARY. 

The  chief  characteristic  features  exhibited  by  individuals  of 
the  six  different  crystal  systems  which  will  prove  of  assistance 
in  microchemical  analysis  may  be  summarized  as  follows: 

ISOMETRIC    SYSTEM    (Cubic  System). 

The  three  crystallographic  axes  are  all  at  right  angles.  Each  axis  is  one  of  four- 
fold symmetry.     All  axes  are  of  like  value,  hence  any  axis  may  be  made  the  c-axis. 

Cleavage  usually  parallel  to  the  faces  of  the  crystal  and  symmetrical  with  refer- 
ence to  the  crystallographic  axes. 

Optically  isotropic,  hence  there  is  no  change  between  crossed  nicols.  No  inter- 
ference figures. 

A  single  refractive  index,  independent  of  direction. 


CRYSTALS   UNDER   THE   MICROSCOPE  181 

TETRAGONAL    SYSTEM. 

Two  equal  horizontal  crystallographic  axes  at  right  angles  to  each  other  and  to 
the  vertical.  Vertical  or  c-axis  either  longer  or  shorter  than  the  other  two.  c-axis 
is  one  of  fourfold  symmetry.     Each  horizontal  axis  is  one  of  twofold  symmetry. 

Interaxes  (lines)  bisecting  the  interaxial  angles  between  a-  and  b-axes  may  also 
serve  as  subordinate  axes  of  symmetry. 

Cleavage,  rectangular. 

Uniaxial. 

Optic  axis  coincident  with  c-axis.  Hence  in  one  position  isotropic;  in  other  two, 
parallel  extinction. 

Crystals  four-sided  or  eight-sided  or  lath-shaped  or  six-sided.  Four-  or  eight- 
sided  crystals  isotropic  (seen  on  end).  Crystals  lying  on  their  side  give  parallel 
extinction. 

Interference  figure:  symmetrical  cross  with  concentric  rings. 

Index  of  refraction,  e  in  direction  parallel  to  optic  axis;  w  index  in  the  plane 
normal  to  the  optic  axis. 

HEXAGONAL    SYSTEM. 

Vertical  or  c-axis  is  at  right  angles  to  the  three  horizontal  axes  at  their  point 
of  intersection.  Horizontal  axes  intersect  at  angles  of  6o°.  c-axis  may  be  longer 
or  shorter  than  the  horizontal  and  is  an  axis  of  sixfold  symmetry.  Each  hori- 
zontal axis  is  one  of  twofold  symmetry. 

Interaxes  may  serve  as  subordinate  axes  of  symmetry. 

Cleavage  lines  usually  intersect  at  angles  of  6o°. 

Uniaxial. 

Optic  axis  coincident  with  c-axis. 

Crystals  three-sided  or  six-sided,  or  long  rectangles  showing  three  faces.  Three- 
angled  and  six-angled  forms  usually  isotropic  (seen  endwise).  Long  crystals 
lying  on  their  sides  exhibit  parallel  extinction. 

Interference  figure:  symmetrical  black  cross  with  concentric  spectrum  colored 
rings.     Tetartohedral  crystals  are  circular  polarizing. 

Indices  of  refraction  have  same  relations  as  in  tetragonal  system. 

ORTHORHOMBIC    SYSTEM. 

Three  axes  at  right  angles  to  each  other,  of  unequal  length.  Each  axis  is  one 
of  twofold  symmetry.     Any  axis  may  be  made  the  vertical. 

Cleavage  in  direction  of  diametral  planes. 

Biaxial. 

Optic  axes:  since  any  crystallographic  axis  according  to  convenience  may  be 
made  the  c-axis  no  relationship  may  be  formulated  between  the  optic  and  crys- 
tallographic axes. 

Extinction  parallel1  in  all  three  positions  of  the  crystals. 

Three  indices  of  refraction,  least  index  in  direction  of  greatest  elasticity,  greatest 
index  in  direction  of  least  elasticity. 

MONOCLINIC    SYSTEM. 

Three  axes  of  unequal  length.  The  a-axis  and  c-axis  are  oblique  to  each  other. 
The  b-axis  is  perpendicular  to  the  other  two  at  their  point  of  intersection.  The 
b-axis  is  an  axis  of  twofold  symmetry. 

1  In  biaxial  crystals  complete  extinction  is  obtained  only  with  monochromatic 
light. 


182  ELEMENTARY   CHEMICAL   MICROSCOPY 

Cleavage  dependent  upon  crystal  species. 
Biaxial. 

Extinction  parallel  in  two  positions,  oblique  in  the  third.     (This  does  not  apply 
to  sections  through  a  crystal.) 
Three  indices  of  refraction. 

TRICLINIC    SYSTEM. 

Three  crystallographic  axes,  all  of  unequal  length  and  oblique  to  one  another. 
There  are  no  axes  of  symmetry. 

Cleavage  dependent  upon  crystal  species. 

Biaxial. 

Extinction  oblique  in  all  three  positions. 

Three  indices  of  refraction. 

EXPERIMENTS    DEALING    WITH  CRYSTAL  FORMS  AND  OPTICAL 

PROPERTffiS. 

Center  the  stage  and  test  graduations  and  accuracy  of  polar- 
izing apparatus. 

Study  the  following  salts,  make  sketches  and  note  their  be- 
havior between  crossed  nicols: 

Isometric  System. 

Sodium  chloride;    potassium  iodide;    barium  nitrate;    ammonia  alum;    chrome 
alum;  arsenic  trioxide;   sodium  chlorate  (circular  polarization). 
Hexagonal  System. 

Lead  iodide;  iodoform;   cadmium  iodide;   normal  sodium  phosphate;  strontium 
chloride;   strontium  antimonyl  tartrate;   sodium  nitrate. 
Tetragonal  System. 

Potassium    arsenate;     mercuric    cyanide;     potassium    copper   chloride;     urea; 
strychnine  sulphate. 
Orthorhombic  System. 

Ammonium  sulphate;    mercuric  chloride;    potassium  antimonyl  tartrate;    po- 
tassium nitrate;  potassium  sulphate;  sodium  nitroprusside;  zinc  sulphate;  uranyl 
acetate. 
Monoclinic  System. 

Potassium  ferrocyanide;    potassium  ferricyanide;    sodium  ferric  oxalate;    am- 
monium   persulphate;     potassium   chlorate;     barium   chloride;     nickel   chloride; 
tartaric  acid;  saccharose. 
Tri clinic  System. 

Copper  sulphate;   potassium  bichromate;   potassium   persulphate;   boric  acid; 
manganous  sulphate. 
Plcochroic  Salts. 

Copper  acetate;  iodoquinine  sulphate;  potassium  (or  sodium)  ferric  oxalate; 
potassium  cobalt  sulphate;   silver  bichromate;  potassium  chromium  oxalate. 

In  a  watch  glass  place  a  few  drops  of  benzene,  add  a  few  crystals  of  quinone, 
stir  until  dissolved.  Add  a  few  crystals  of  resorcin,  stir.  Remove  a  drop  to  a 
slide  and  allow  it  to  deposit  crystals  by  spontaneous  evaporation.  The  crystals 
will  be  found  to  be  strongly  pleochroic. 

.  Note.     The  experiments  outlined  above  have  dealt  with  normal  well-formed 
^crystals  whose  habits  could  be  ascertained  with  but  little  difficulty.     Abnormal 


CRYSTALS   UNDER   THE   MICROSCOPE  183 

or  imperfect  forms  are,  however,  very  common  in  the  case  of  many  compounds  and 
especially  where  several  substances  are  present  in  solution. 

The  presence  of  colloids  in  the  solution  will  in  most  cases  either  wholly  inhibit 
crystallization  or  will  so  greatly  modify  the  form  of  the  crystals  separating  as  to 
prevent  their  being  identified  as  the  substance  which  has  crystallized  out.  In  the 
following  experiments  the  effect  of  colloids  will  be  seen. 

EXPERIMENTS   DEALING   WITH   ABNORMAL  CRYSTALLIZATIONS. 

1.  Dissolve  two  or  three  fragments  of  sodium  chloride  in  water,  concentrate 
and  examine  the  crystals  separating.  Add  a  drop  of  a  concentrated  solution  of 
gum  arabic,  warm  gently  until  all  the  salt  has  dissolved,  concentrate  to  crystalli- 
zation at  as  low  a  heat  as  possible  and  note  well  the  change  in  the  character  of  the 
crystals  separating.  Instead  of  cubes  and  rectangular  plates  skeleton  forms  and 
dendritic  masses  appear. 

2.  Try  a  similar  experiment  using  ammonium  chloride.  In  neither  case  can 
well-defined  crystals  be  obtained,  but  the  appearance  of  the  dendritic  masses  has 
changed  under  the  influence  of  the  colloid. 

3.  Place  a  tiny  drop  of  water  on  a  slide  on  a  piece  of  white  paper,  add  ferric 
chloride  until  a  distinct  yellow  color  is  obtained.  Now  add  two  or  three  frag- 
ments of  ammonium  acetate,  stir  gently  until  dissolved,  but  do  not  heat.  Cr\s- 
tallization  will  soon  set  in.  The  crystals  of  ammonium  chloride  now  separating 
will  be  well-formed  cubes  or  the  skeletons  of  cubes. 

4.  Try  crystallizing  mercuric  chloride  in  the  presence  of  gum  arabic. 

5.  Place  next  to  a  drop  of  a  solution  of  lead  nitrate  a  solution  of  potassium 
iodide.  Cause  the  drops  to  flow  together.  Note  that  lead  iodide  is  formed  in 
shining  iridescent  plates.  To  a  drop  of  a  solution  of  lead  nitrate  add  a  fragment 
of  gelatin,  warm  gently  to  dissolve  the  gelatin.  Place  a  drop  of  potassium  iodide 
solution  next  to  the  test  drop  just  prepared  and  cause  the  iodide  solution  to  flow 
into  the  test  drop.  It  will  be  evident  that  lead  iodide  has  been  formed  because  of 
the  yellow  zone  at  the  point  of  contact,  but  no  crystals  will  separate. 

6.  To  a  drop  of  olive  or  cottonseed  oil  on  a  slide  add  stearic  acid  and  warm 
gently  until  the  drop  is  clear.  On  cooling,  radiating  masses  of  thin  plates  separate. 
Examine  between  crossed  nicols. 

7.  Place  a  few  fragments  of  sulphonal  upon  a  slide,  lay  a  cover  glass  upon  the 
material  and  heat  until  the  sulphonal  melts.  A  very  thin  molten  film  of  the  com- 
pound results,  which  crystallizes  on  cooling.     Examine  between  crossed  nicols. 

8.  Dissolve  a  very  minute  amount  of  barium  chloride  at  the  corner  of  a  slide; 
add  a  fragment  of  sodium  acetate.  Dissolve  a  fragment  of  oxalic  acid  in  a  drop 
of  water  close  to  the  drop  of  barium  chloride  solution.  Cause  the  oxalic  acid  to 
flow  into  the  other  drop.  In  a  few  seconds  large  branching  aggregates  in  the  form 
of  radiating  bundles  and  sheaves  of  fibrous  needles  of  barium  oxalate  will  be  seen. 

Make  a  fresh  solution  drop  of  barium  chloride;  add  sufficient  ferric  chloride  to 
give  the  drop  a  distinct  yellow  color.  Now  add  sodium  acetate  as  before;  stir 
until  dissolved.  The  drop  should  now  acquire  a  reddish  tint.  Into  this  test  drop 
cause  oxalic  acid  to  flow.  Long  hair-like  crystals  (trichites)  separate  instead  of 
branching  aggregates. 

p.  Prepare  a  drop  of  an  almost  saturated  solution  of  chromium  chloride.  Add  a 
little  solid  mercuric  chloride  and  warm  gently  until  dissolved.  Concentrate  to 
crystallization.     Trichiten  crystals  of  the  double  chloride  separate. 

10.  Prepare  a  solution  drop  of  sodium  asparaginate.  Try  to  obtain  crystals  by 
concentration.  Start  crystallization  by  "seeding;"  that  is,  crush  in  the  drop  the 
smallest  possible  piece  of  the  salt. 


CHAPTER  IX. 

THE  DETERMINATION  OF  REFRACTIVE  INDEX  BY  MEANS 

OF  THE  MICROSCOPE. 

All  transparent  and  translucent  objects  when  immersed  in 
liquids  yield  images  in  the  microscope  which  are  bounded  by 
dark  lines  or  bands  or  which  appear  to  be  surrounded  by  a  colored 
fringe  or  halo.  The  width  or  thickness  of  these  dark  or  colored 
contours  depends  upon  the  magnitude  of  the  difference  between 
the  refractive  indices  of  the  two  phases  (the  solid  and  the  liquid), 
upon  the  dispersive  power  of  each  and  upon  the  method  of  illumi- 
nation employed. 

Contour  bands  appear  when  the  refractive  index  of  the  solid 
is  either  greater  or  less  than  that  of  the  liquid  in  which  the  solid 
is  immersed.  As  the  index  of  refraction  of  the  solid  approaches 
closer  and  closer  to  that  of  the  liquid  the  dark  bands  decrease  in 
prominence,  and  finally  vanish  when  both  object  and  liquid  have 
the  same  refractive  index.  If  both  have  also  the  same  dispersive 
power,  the  same  light-absorbing  power  and  the  same  color,  the 
object  will  be  invisible  in  the  liquid.  But  complete  disappear- 
ance is  impossible  in  practice  since  these  conditions  can  never 
be  all  fulfilled  and  since  moreover  it  is  next  to  impossible  to  obtain 
crystals  or  other  solids  which  are  so  perfect  as  to  be  free  from  air 
bubbles,  fractures  or  cleavage  planes  or  which  contain  no  occlu- 
sions of  dirt,  of  mother  liquor  or  of  foreign  salts.  The  vanishing 
of  the  black  lines  is  therefore  the  criterion  upon  which  we  must 
depend  for  an  indication  that  the  solid  and  the  liquid  have  the 
same  index  of  refraction. 

It  is  evident,  that,  given  a  series  of  liquids  of  known  refrac- 
tive index,  if  a  solid  of  unknown  index  be  immersed  in  these, 
one  after  another,  until  the  black  contours  bounding  the  image 
just  disappear,  the  index  of  this  particular  liquid  is  the  index 
sought  of  the  solid. 

In  like  manner  if  we  have  a  series  of  crystals,  or  fragments  of 

184 


THE   DETERMINATION   OF   REFRACTIVE   INDEX  1 85 

transparent  solids  whose  indices  of  refraction  we  know,  it  is 
possible  to  roughly  ascertain  the  index  of  a  given  liquid. 

The  index  o*f  refraction  is  a  constant  for  any  given  substance 
of  definite  composition.  Its  determination  often  affords  a  ready 
means  of  identification  or  differentiation  and  in  many  instances 
is  in  fact  the  only  simple  means  at  our  command  for  the  recogni- 
tion of  a  compound. 

Although  the  identification  of  compounds  through  determi- 
nations of  their  refractive  indices  by  the  immersion  method  and 
the  microscope  has  long  been  practiced  by  mineralogists,  penol- 
ogists and  microscopists,  it  is  only  within  the  last  few  years 
that  chemists  have  awakened  to  the  value  of  the  data  so  easily 
obtained. 

A  determination  of  the  refractive  index  is  of  special  value  in 
the  qualitative  analysis  of  soils,  sands,  mineral  fragments,  etc., 
in  the  study  of  crystalline  residues,  in  the  differentiation  of 
isomeric  compounds  and  in  the  study  of  materials  which,  although 
pure,  cannot  properly  be  separated  from  foreign  matter. 

In  order  to  determine  the  refractive  index  of  crystalline  solids 
we  may  proceed  as  described  below: 

Determination  of  the  Refractive  Index  of  Isotropic  Substances. 
—  One  or  more  tiny  fragments  or  crystals  of  the  material  are 
placed  upon  a  clean  slide,  a  small  drop  of  a  liquid  of  known 
refractive  index  is  placed  upon  a  small,  scrupulously  clean  cover 
glass  and  the  cover  with  its  drop  is  inverted  and  laid  upon  the 
solid  under  investigation,  care  being  observed  in  lowering  the  glass 
with  its  drop  of  liquid  to  avoid  the  formation  of  air  bubbles.  Place 
the  preparation  under  the  microscope  with  the  Abbe  condenser1 
raised  as  high  as  it  will  go.  Focus  with  a  32  millimeter  or  16 
millimeter  objective.  Under  these  conditions  the  preparation 
will  probably  be  flooded  with  light.  Close  the  diaphragm  two- 
thirds  or  even  more.  If  the  crystal  fragments  are  not  now  clear 
and  distinct  with  sharply  defined  contours  lower  the  condenser 
a  trifle,  but  only  a  trifle.  It  is  of  course  possible  that  in  selecting 
the  liquid  one  of  the  same  refractive  index  as  that  of  the  solid 
may  have  been  chosen;   this  is,  however,  very  unlikely. 

1  These  directions  refer  specifically  to  the  Chemical  Microscope  described  on  p.  19. 


1 86  ELEMENTARY  CHEMICAL  MICROSCOPY 

Contour  bands  appear  whether  the  solid  has  either  a  higher  or 
a  lower  index  than  the  surrounding  liquid.  The  next  step  must 
therefore  be  to  ascertain  whether  it  is  higher  or  lower  than  the 
liquid  employed.  This  is  accomplished  by  slowly  raising  the 
microscope  tube  by  means  of  the  coarse  adjustment,  at  the  same 
time  closely  observing  the  change  in  appearance,  direction  of 
motion  or  change  in  color  of  the  contour  bands  and  halo-like  band 
of  light  bounding  the  crystal  fragments.  When  the  solid  pos- 
sesses a  higher  index  than  that  of  the  liquid,  the  contours  are 
usually  dark  and  well  defined  with  a  halo  or  band  of  light  within 
the  black  bands;  as  the  microscope  tube  is  raised  this  band  of 
light  wall  appear  to  move  inward,  i.e.,  toward  the  center  of  the  solid. 
If,  on  the  other  hand,  the  solid  possesses  a  lower  index  of  re- 
fraction, the  black  contours  are  relatively  weak,  with  the  bright 
halo  outside  the  black  bands,  and  upon  raising  the  objective  the 
band  of  light  or  bright  halo  appears  to  move  outward  or  away  from 
the  center.  This  difference  in  behavior  is  due  to  the  fact  that 
when  the  fragment  has  a  higher  refractive  index  than  the  liquid 
it  causes  the  rays  leaving  it  to  converge,  but  if  the  solid  has  a 
lower  refractive  index  the  emerging  rays  are  divergent.  In  order 
to  obtain  the  best  results  by  this  method,  always  screen  the  prepa- 
ration upon  the  stage  with  the  hand;  thus  none  but  transmitted 
light  rays  can  enter  the  objective. 

By  employing  oblique  instead  of  axial  light  it  becomes  still 
easier  !  to  determine  whether  the  solid  possesses  a  higher  or  a 
lower  refractive  index  than  the  liquid  in  which  it  is  immersed. 

Before  considering  the  method  of  procedure  in  this  case  let  us 
study  several  simple  yet  instructive  experiments.2 

Place  a  small  drop  of  mucilage  or  thin  gum  upon  an  object 
slide,  beat  it  with  a  knife  blade  until  full  of  air  bubbles.  Cover 
with  a  cover  glass  and  place  upon  the  stage  of  the  microscope. 
Use  an  8  millimeter  objective  and  center  a  tiny  air  bubble  whose 
image  appears  to  be  not  over  i  to  2  millimeters  in  diameter. 
Focus  sharply.  The  image  obtained  will  consist  of  a  tiny  disk 
of  light  surrounded  by  a  black  ring.     We  are  here  dealing  with  a 

1  Schroeder  van  der  Kolk,  Zeit.  anal.  Chem.,  38  (1899),  615. 

2  Gage,  The  Microscope  (1908),  p.  106,  10th  ed.,  Ithaca,  N.  Y. 


THE   DETERMINATION   OF   REFRACTIVE   INDEX 


I87 


sphere  of  less  refractive  index  (air  n  =  1)  surrounded  by  a  liquid 
of  higher  refractive  index  (gum  solution  or  water  n  =  i-3+). 
Remove  the  condenser  and  slowly  swing  the  mirror  to  one  side, 
looking  into  the  microscope  at  the  same  time.  As  the  light  be- 
comes oblique  the  bright  disk  in  the  image  of  the  air  bubble 
moves  in  the  opposite  direction  from  the  movement  of  the  mirror. 
Move  the  mirror  back  and  the  reverse  phenomenon  will  be 
observed.  When  the  light  is  exactly  axial  the  bright  spot  will 
be  exactly  at  the  center  of  the  black  circle.  This  constitutes 
one  of  the  simplest  and  best  tests  for  axial  light  that  we  possess. 
Now  slowly  raise  the  objective;  the  bright  disk  will  be  seen  to 
grow  larger  and  larger  and  the  black  ring  will  appear  to  move 
outward  and  the  disk  will  become  indistinguishable  before  the 
surrounding  ring  vanishes. 

Take  a  drop  of  water  and  mix  very  thoroughly  with  it  by  gentle 
beating  a  tiny  droplet  of  oil.  There  are  thus  obtained  tiny 
spheres  of  oil  of  a  refractive  index  higher  (oil  n  =  1.4+)  than  that 
of  the  surrounding  liquid  (water  n  =  1.33).     Again  we  obtain 


Fig.  106.     Oil  Globule  and  Air  Bubble  illuminated  with  Axial  Light.   (Gage.) 


as  the  image  of  a  tiny  globule,  a  bright  disk  surrounded  by  a  dark 
ring.  With  axial  light  this  disk  is  concentric;  with  oblique  light 
eccentric.  As  the  mirror  is  swung  aside  the  disk  of  light  in  the 
image  appears  to  move  in  the  same  direction  as  the  mirror.  Upon 
raising  the  objective  the  disk  of  light  grows  smaller  and  smaller, 
the  black  annular  contour  band  appears  to  move  inward  and  the 
bright  spot  is  the  last  to  disappear.  These  phenomena  are 
readily  interpreted  by  referring  to  the  diagrams,  Figs.  106  and 
107.     With  air,  n  <  liquid,  the  emerging  rays  are  divergent; 


i88 


ELEMENTARY   CHEMICAL   MICROSCOPY 


with  oil,  n  >  liquid,  the  emergent  rays  are  converging.  In  Fig. 
107  the  solid  line  arrows  indicate  the  direction  of  the  moving 
mirror,  while  the  dotted  line  arrows  that  of  the  corresponding 
direction  of  movement  of  the  disk  of  light  in  the  image.     These 


Fig.  107.     Oil  Globule  and  Air  Bubble  illuminated  with  Oblique  Light.   (Gage.) 

diagrams  indicate  the  behavior  of  the  light  rays,  but  in  the  image 
in  the  microscope  positions  and  directions  are  reversed;  hence 
the  phenomena  observed  are  those  described  above. 

It  thus  appears  that  under  oblique  illumination  the  contour 
bands  are  heavier  or  darker  on  one  side  of  the  image  of  the  object 
than  on  the  other,  the  particular  side  which  is  darker  depending 
upon  the  difference  in  the  indices  of  object  and  mounting  medium 
and  the  direction  of  the  illuminating  rays.  Advantage  is  taken 
of  these  facts  to  determine  by  means  of  oblique  light  whether  an 
object  whose  refractive  index  is  sought  has  a  higher  or  lower 
index  than  that  of  the  test  liquid  in  which  it  is  immersed.  Ob- 
lique light *  is  obtained  by  swinging  the  mirror  to  one  side  when 
no  condenser  is  employed,  or  by  sliding  a  piece  of  black  paper 
or  card  just  below  the  condenser  or  by  holding  a  ringer  just  below 
the  condenser  so  as  to  cut  off  about  one-half  the  lower  aperture. 

In  the  chemical  microscope  slide  a  piece  of  stiff  black  paper 
between  the  condenser  and  the  ring  attached  to  its  lower  part. 
The  preparation  on  the  stage  will  then  be  illuminated  by  oblique 

1  See  Wright,  Oblique  Illumination  in  Petrographic  Microscopic  Work;  Amer. 
J.  Sci.  (4)  36  (1013),  63. 


THE   DETERMINATION   OF   REFRACTIVE   INDEX 


189 


light.  The  phenomena  resulting  can  best  be  understood  by- 
consulting  Figs.  108  and  109,  in  which  the  indicated  directions  of 
the  passage  of  light  rays  have  been  greatly  exaggerated.  The 
crystal  H  has  a  higher  refractive  index  than  the  liquid  surround- 


\  v  v\  ^ 


Fig.  108.     Contour  Bands  in  Half  Shadow- 
Illumination. 


Fig.  109.      Contour  Bands  in 
Half  Shadow  Illumination. 


ing  it;  the  rays  passing  through  are  therefore  convergent,  but 
only  those  at  the  left  can  enter  the  objective  O;  hence,  the  left 
side  is  bright  and  the  right  side  dark.  But  in  the  case  of  the 
crystal  L  whose  index  is  less  than  that  of  the  liquid  the  emerg- 
ing rays  diverge,  yet  here  again  only  part  of  the  rays  can  enter 
the  objective  O;  in  this  instance  those  on  the  right;  thus  the 
right  side  is  bright:  the  left  dark  or  in  other  words,  the  opposite 
of  the  phenomena  observed  with  crystal  H. 

Conducting  our  observations  with  the  condenser  only  very 
slightly  lowered  and  the  paper  diaphragm  inserted  from  the  left 
until  the  dark  shadow  extends  approximately  to  the  center  of 
the  field,  the  phenomena  seen  will  be  as  indicated  in  Fig.  109. 
The  crystal  H  of  higher  index  than  the  liquid  appears  dark  on 
the  dark  side  of  the  field  and  bright  on  the  light  side  of  the 
field;  but  the  crystal  fragment  L  of  lower  index  than  the  liquid 
appears  bright  on  the  dark  side  of  the  field  and  dark  on  the  bright 
side  of  the  field.  This  is  as  it  should  be  from  Fig.  108,  since  in 
the  image  formed  in  the  microscope  the  directions  are  reversed. 

If  we  now  lower  the  condenser  a  reversal  of  all  the  above 
phenomena  takes  place.  It  is  therefore  always  wise  to  check 
the  results  recorded  with  condenser  raised  by  lowering  the  con- 


190  ELEMENTARY    CHEMICAL   MICROSCOPY 

denser;  moreover  the  phenomena  are  much  more  distinct  with 
lowered  condenser. 

There  is  little  chance  for  an  error  of  judgment  if  the  student 
will  start  with  condenser  raised  and  stopped  down,  and  first 
slowly  raise  the  objective,  noting  the  direction  of  apparent  move- 
ment of  the  contour  bands  or  halo.  Next  test  with  oblique  light 
and  note  the  relative  position  of  the  dark  contours  with  respect 
to  the  dark  half  of  the  field  and  finally  lower  the  condenser  and 
test  again  with  oblique  light.  All  three  of  the  sets  of  observa- 
tions should  be  in  accord.  The  student  should  also  learn  to  use 
a  finger  below  the  condenser  to  obtain  oblique  illumination  and 
thus  save  time. 

Since  most  of  the  liquids  employed  for  the  determination  of 
refractive  index  by  the  immersion  method  have  a  greater  dis- 
persive power  than  the  solids,  at  the  end  point  in  the  immersion 
method  the  images  usually  appear  surrounded  by  colored  fringes. 
The  conditions  which  usually  obtain  are  that  when  the  liquid  and 
solid  have  the  same  refractive  index  for  yellow-green  rays,  the 
liquid  will  have  a  higher  n  for  blue  rays  than  the  solid  but  the 
solid  will  have  a  higher  n  for  red  rays  than  the  liquid.  It  follows 
that  the  emerging  red  rays  will  be  convergent  as  diagramed  in 
S,  Fig.  108,  while  the  emerging  blue  rays  will  be  divergent.1  No 
dark  contour  bands  will  be  sufficiently  prominent  to  be  noticeable, 
but  the  image  will  exhibit  a  bluish  fringe  on  the  outside  and  a 
reddish  fringe  on  the  inside,  or  with  oblique  fight  bluish  on  one 
side,  reddish  on  the  other.  Raising  the  objective  will  cause  the 
red  fringe  to  move  inward  and  the  blue  fringe  outward.  It  is 
evident  that  this  color  dispersion  phenomenon  enables  us  to  still 
further  assure  ourselves  when  we  have  found  the  liquid  having 
the  same  n  as  that  of  the  solid  under  examination. 

When  in  the  course  of  the  experiments  a  marked  color  fringe 
is  seen  with  the  absence  of  black  bands,  the  point  has  been 
reached  in  which  liquid  and  solid  have  the  same  refractive  index 
for  light  rays  of  medium  wave  length.  To  obtain  more  accurate 
results  recourse  must  be  had  to  monochromatic  light. 

In  preparing  a  series  of  liquids  of  regularly  differing  refractive 
1  Wright,  Amer.  J.  Sci.,  loc.  cit. 


THE   DETERMINATION   OF  REFRACTIVE   INDEX  191 

indices  for  use  in  this  immersion  method,  it  is  advantageous  to 
select  those  having  a  slightly  greater  color  dispersion  than  will 
be  found  in  the  solids  to  be  tested.  But  highly  dispersive  liquids 
must  be  avoided  since  the  color  bands  or  halos  are  then  so  marked 
as  to  seriously  interfere  with  the  recognition  of  dark  contours. 

Having  ascertained  as  described  above  whether  the  crystal 
fragment  has  a  higher  or  a  lower  index  than  that  of  the  liquid 
first  tried,  and  thus  in  which  direction  to  proceed,  a  second  liquid 
whose  index  is  probably  very  much  nearer  that  of  the  solid  is 
chosen.  The  first  liquid  is  carefully  removed  by  absorbing  it 
with  a  bit  of  filter  paper,  a  drop  of  the  liquid  next  to  be  applied  is 
added  and  allowed  to  flow  completely  around  the  crystal;  after 
standing  a  few  moments  this  is  removed  as  before  and  a  new  por- 
tion added.  The  preparation  is  tested  by  raising  the  objective 
and  by  the  half-shadow  method  to  learn  whether  the  solid  or  the 
liquid  has  the  higher  index.  The  process  is  repeated  until  the 
proper  liquid  has  been  found.  In  making  the  trials  add  first  a 
liquid  of  a  higher  then  one  of  lower  value.  When  sufficient  solid 
material  is  available  it  will  be  found  that  time  will  be  saved  and 
much  more  reliable  data  obtained  if  an  entirely  new  preparation 
is  made  with  each  liquid.  This  also  avoids  wasting  valuable 
liquids. 

At  the  end  of  the  chapter  will  be  found  tables  :  of  liquids  for 
use  in  the  determination  of  refractive  indices.  In  Table  IV  will 
be  found  the  indices  of  isometric  crystals  useful  in  estimating 
the  refractive  indices  of  liquids. 

If  it  is  found  that  the  index  of  no  liquid  in  a  series  at  hand  cor- 
responds to  that  of  the  crystal  under  observation,  mixtures  of 
two  liquids  may  be  made  and  the  index  of  refraction  of  the  mix- 
ture can  roughly  be  estimated  from  the  formula,2 

»iVi  +  «-2V,  -  n  (Vi  +  V2), 

in  which  Vi  and  V2  are  the  volumes  of  the  two  liquids,  rh  and  th 

1  For  exceptionally  complete  lists  of  media  for  refractive  index  determinations 
see  Johannsen,  Manual  of  Petrographic  Methods. 

2  Schroeder  v.an  der  Kolk,  Mikroskopische  Krystallbestimmung,  Weisbaden, 
1898,  p.  13. 


192  ELEMENTARY   CHEMICAL   MICROSCOPY 

their  respective  indices  of  refraction  and  n  the  refractive  index 
of  the  mixture.  It  is  obvious  that  in  the  preparation  in  this 
manner,  of  liquids  of  intermediate  index  values,  it  is  essential 
that  the  two  liquids  shall  be  miscible  in  all  proportions,  and  that 
no  new  chemical  compound  shall  result  from  the  mixing.  Since 
a  determination  of  refractive  index  may  often  require  a  period  of 
time  sufficiently  long  to  result  in  an  appreciable  loss  of  liquid 
through  evaporation,  the  liquids  chosen  for  mixing  should, 
theoretically,  not  differ  greatly  in  their  boiling  points,  otherwise 
there  is  a  possibility  of  the  concentration  of  the  less  volatile  liquid 
increasing.  It  will  also  be  obvious  that  in  order  to  obtain  suffi- 
ciently exact  calculated  values  from  the  equation  given  above, 
the  liquids  mixed  should  not  have  widely  different  densities.  For 
these  reasons  approximate  boiling  points  and  densities  have  been 
given  in  Table  I.  This  formula  for  calculating  the  index  of  refrac- 
tion of  a  mixture  of  two  liquids  is  based  upon  the  assumption  that 
to  the  final  mixture  each  component  contributes  equally  its  own 
proportional  part  of  the  final  index.  There  seems  to  be  no  sound 
theory  in  support  of  this  assumption  nor  do  the  facts  appear  to 
be  in  accord  with  the  formula.  From  experiments  made  in  the 
Cornell  laboratories,  using  an  Abbe  refractometer  to  determine 
the  refractive  index  of  the  liquids  before  and  after  mixing,  it  was 
found  that  the  calculated  results  were  not  always  dependable  to 
the  second  decimal  place.  The  first  decimal  was  always  correct, 
the  second  usually  so,  but  very  rarely  indeed  would  the  third 
agree  in  calculated  and  found  values.  Mixtures  allowed  to 
stand  several  days  and  again  measured  gave  similar  results, 
showing  that  equilibrium  had  been  reached  when  the  first  obser- 
vations had  been  made. 

Formulas  of  the  kind  given  above  should  not  be  employed  if 
the  same  degree  of  accuracy  in  calculation  is  wanted,  as  the  im- 
mersion method  will  yield  in  practice. 

The  immersion  method  above  described  permits  an  accuracy 
in  the  determination  of  the  refractive  index  within  0.005  ±  but 
with  monochromatic  light  and  more  refined  methods  of  illumi- 
nation an  accuracy  of  0.002  ±  or  even  o.ooi±  may  sometimes 
be  reached. 


THE   DETERMINATION   OF   REFRACTIVE   INDEX  193 

The  Refractive  Index  of  Anisotropic  Substances.  -  -  Crys- 
tals are  either  isotropic  or  anisotropic.  In  isotropic  crystals 
light  rays  are  refracted  to  an  equal  degree,  no  matter  in  what 
direction  through  the  crystals  the  rays  are  sent,  since  the  velocity 
of  transmission  of  light  is  the  same  in  all  directions  through  the 
crystals,  providing  the  crystals  have  not  been  subjected  to 
stresses  or  strains.  In  the  determination  of  the  refractive  indices 
of  isotropic  crystals,  it  is  obvious  that  the  same  value  will  be 
obtained  in  all  directions  through  the  crystals.  In  the  case  of 
anisotropic  crystals,  however,  the  rate  of  transmission  of  light  is 
different  in  different  directions  through  the  crystals.  In  order 
to  better  appreciate  the  influence  of  these  properties  upon  the 
refractive  index,  it  is  necessary  to  briefly  consider  a  few  funda- 
mental facts. 

A  ray  of  light,  when  passing  obliquely  from  one  medium  into 

another  whose  rate  of  transmission  for  light  rays  is  different, 

will  be  deflected  from  its  original  path  according  to  the  equation 

sin.  %        y/ 

- —  =  —. ,  in  which  i  is  the  angle  formed  by  the  incident  ray 

sin  r      V 

and  the  normal,  r  the  angle  formed  by  the  refracted  ray  and  the 
normal,  and  V  and  V  the  velocities  of  the  transmission  of  the 
light  in  the  two  media.  When  the  rays  pass  from  a  medium 
having  a  higher  rate  of  transmission  into  one  of  lesser  rate  the 
deflection  is  toward  the  normal,  but  when  passing  from  a  medium 
with  a  lesser  rate  into  one  of  higher  rate  the  bending  is  away  from 
the  normal.  In  microscopic  work  the  light  rays  are  usually 
passing  from  air  into  a  denser  medium.  If  in  the  above  equation 
we  assign  to  the  velocity  of  light  in  air  the  value  of  1,  the  equation 

becomes  - —  =  — . ,  but  — —  is  the  expression  for  the  index  of 
sm  r      \  sin  r 

refraction,  from  which  it  appears  that  the  refractive  index  is 
inversely  proportional  to  the  velocity  of  the  transmission  of  light 
in  the  medium.  Since  in  anisotropic  crystals,  the  rate  of  trans- 
mission of  light  rays  differs  according  to  the  direction  through 
the  crystal  in  which  the  rays  are  sent,  it  is  obvious  that  the  re- 
fractive index  of  an  anisotropic  crystal  cannot  be  expressed  by  a 
single  value  and  further,  that  of  the  several  values  given  by  a 


194  ELEMENTARY   CHEMICAL   MICROSCOPY 

doubly  refracting  crystal,  the  greatest  index  will  be  found  in  the 
direction  through  the  crystal  of  the  lowest  rate  of  light  trans- 
mission and  the  smallest  index  in  the  direction  of  the- highest 
rate  of  light  transmission.  In  other  words,  different  values  for 
the  index  of  refraction  will  be  obtained  according  to  the  position 
in  which  the  crystals  he  upon  the  stage  of  the  microscope. 

Crystals  belonging  to  the  tetragonal  and  hexagonal  systems 
(uniaxial  crystals)  possess  two  indices.  Crystals  belonging  to 
the  orthorhombic,  monoclinic,  and  triclinic  systems  (biaxial 
crystals)  have  three  indices. 

In  uniaxial  crystals  one  value  corresponds  to  that  given  by  the 
ordinary  ray  and  the  other  to  that  given  by  the  extraordinary 
ray.  The  first  value  is  found  in  that  direction  through  the  crys- 
tal where  the  light  vibrations  are  transmitted  transverse  to  the 
vertical  crystallographic  (and  in  this  case  optical)  axis  and  is 
designated  by  the  Greek  letter  w;  the  second  value  is  observed 
when  light  is  transmitted  through  the  crystal  parallel  to  the  verti- 
cal axis.  This  index  is  designated  by  the  Greek  letter  e.  The 
double  refraction  of  uniaxial  crystals  is  said  to  be  strong  when  u 
is  greater  than  e,  and  weak  when  the  reverse  is  found.  When  the 
refractive  index  o  is  greater  than  e,  the  crystal  is  said  to  be  opti- 
cally negative  and  when  less  than  e,  optically  positive.  Some 
crystallographers  prefer  to  designate  the  two  refractive  indices 
by  the  letters  a  and  7.  In  this  case  7  —  a  expresses  the  strength 
of  double  refraction  and  when  a  is  greater  than  7  the  crystal  is 
optically  negative.1 

In  biaxial  crystals  three  different  values  for  the  rate  of  light 
transmission  can  be  found,  or  in  other  words  biaxial  crystals  have 
three  axes  of  elasticity  or  directions  of  vibration;  the  axis  of 
maximum  rate  of  vibration  transmission  is  commonly  designated 
by  the  German  letter  a ;  that  of  the  minimum  vibration  by  c  and 
the  intermediate  axis  by  b.  Since  there  are  three  axes  of  elas- 
ticity, three  different  values  for  the  index  of  refraction  may  be 
obtained,  the  smallest  value  a  in  the  direction  of  the  maximum 

1  In  order  to  be  sure  of  the  values  for  w  and  e,  a  number  of  different  crystals 
should  be  tried  out.  a>  will  be  constant  in  all  of  them,  e  will  differ  slightly  accord- 
ing to  the  position  of  the  crystals. 


THE   DETERMINATION   OF   REFRACTIVE    INDEX  195 

axis  a,  the  greatest  value  7  in  the  direction  of  the  axis  c  and  an 
intermediate  value  (3  in  the  direction  of  the  b  axis.  The  double 
refraction  of  the  crystal  will  be  strong  or  weak  according  to  how 
much  greater  7  is  than  a.  To  determine  whether  a  biaxial  crys- 
tal is  optically  positive  or  negative  requires  other  data  than 
refractive  indices. 

In  uniaxial  crystals  the  determination  of  which  index  is  co 
and  which  e  is  comparatively  simple  since  e  coincides  with 
the  crystallographic  c  axis;  but  in  the  case  of  biaxial  crystals  it 
is  seldom  that  a  chemist  possesses  either  the  knowledge  or  a 
microscope  sufficiently  well  equipped  to  definitely  locate  the 
different  axes  of  elasticity,  since  their  directions  are  indicated  by 
neither  the  crystallographic  nor  the  optical  axes.  For  this 
reason  it  is  wiser  for  the  chemist-analyst  to  follow  the  methods  of 
Kley,1  Bolland  2  and  others,  and  record  values  as  obtained  in  the 
method  given  below. 

Swing  the  polarizer  in  place,  having  first  removed  all  condens- 
ing lenses.  Place  upon  the  stage  an  object  slide  carrying  the 
crystals  or  crystal  fragments  to  be  examined  immersed  in  a 
liquid  of  known  refractive  index  and  covered  with  a  tiny  thin 
cover  glass.  Place  the  analyzer  over  the  eyepiece  (or  slide  it 
into  the  tube  if  an  instrument  of  this  type  is  used)  and  set  the 
graduated  circles  of  both  prisms  at  zero  so  that  their  planes  of 
vibration  are  crossed.  Turn  the  stage  of  the  microscope  until 
the  crystal  selected  for  observation  extinguishes;  remove  the 
analyzer.  Ascertain  by  raising  the  objective  whether  the  index 
of  the  crystal  is  greater  or  less  than  the  liquid;  check  results  by 
oblique  light  by  placing  the  finger  part  way  across  the  opening 
of  the  polarizer.  Substitute  one  liquid  after  another  until  the 
refractive  index  of  the  crystal  is  ascertained,  being  very  careful 
not  to  alter  the  position  of  the  crystal.  If  the  crystal  is  moved 
replace  the  analyzer  and  readjust  the  crystal  to  the  position  of 
extinction.  Read  the  position  of  the  crystal  as  indicated  on  the 
circumference  of  the  stage  and  rotate  the  stage  so  as  to  turn  the 
crystal  exactly  90  degrees  to  its  position  of  extinction  and  proceed 

1  Kley,  Zcit.  anal.  Chem.,  43  (1904),  160. 

2  Bolland,  Monats.,  29  (1908),  991;   31  (1910),  387- 


196 


ELEMENTARY   CHEMICAL   MICROSCOPY 


with  the  determination  of  the  refractive  index  just  as  before. 
The  two  values  obtained  will,  in  the  case  of  uniaxial  crystals,  be 
the  indices  e  and  w.  When  dealing  with  biaxial  crystals  in  order 
to  use  the  values  in  Bolland's  tables  first  set  the  crystal  so  that 
its  prism  edge  lies  parallel  to  a  plane  passing  through  the  short 
diagonal  of  the  polarizing  nicol.  Next  determine  the  index  for  a 
position  at  90  degrees  to  the  first.  If  a  third  value  can  be  found, 
determine  it.  If  the  values  for  a  and  7  are  wanted,  determine 
the  values  for  a  very  large  number  of  fragments;  the  minimum 
value  will  be  a  and  the  maximum  7. 

Determination  of  the  Refractive  Index  of  a  Liquid  by  the 
Method  of  the  Displacement  of  Images.  -  -  When  an  object  is 
viewed  through  a  liquid  from  a  point  in  a  line  normal  to  the  plane 
in  which  the  object  lies,  the  image  observed  will  appear  to  lie  in 
a  plane  above  that  of  the  object,  the  amount  of  displacement  being 
dependent  upon  the  refractive  index  of  the  interposed  medium.1 


If,  therefore,  we  place  a  liquid  in  a  cell  of  depth  DD'  (Fig.  no) 
and  measure  the  amount  of  displacement  of  image  00'  of  a  mark 
at  0  upon  the  lower  surface  of  the  glass  slide,  the  index  of  re- 

DD' 

fraction  n  will  be  found  from  the  equation  n  =fT777' 

1  This  method  is  very  old  and  is  generally  known  as  the  Due  de  Chaulnes 
Method,  having  been  described  by  him  in  1767-1770. 

See  also  Sorby,  Chem.  N.,  37  (1878),  151;  Watson,  Physics;  Johannsen,  Manual 
of  Petrographic  Methods. 


THE   DETERMINATION   OF   REFRACTIVE    INDEX  197 

Method.  —  Cement  upon  a  thin  object  slide  of  clear  glass,  a 
ring,  whose  top  and  bottom  are  ground  true  and  parallel.  The 
cell  thus  formed  should  be  approximately  one  millimeter  deep 
and  several  millimeters  in  diameter.  Place  in  the  cell  the  liquid 
whose  refractive  index  is  sought  and  cover  with  a  thin  cover  glass 
of  greater  diameter  than  that  of  the  cell,  as  shown  in  Fig.  no. 

Determine  the  thickness  of  the  liquid  layer  by  means  of  the 
graduations  on  the  fine  adjustment  as  follows:  focus  carefully 
upon  the  upper  surface  of  the  object  slide.  Read  the  position 
of  the  fine  adjustment.  Slide  the  cell  along  until  the  projecting 
cover  glass  is  in  the  field  and  focus  upon  the  under  side  of  the 
cover  glass  and  record  this  value.  This  will  give  the  depth  of 
the  liquid  plus  an  error  due  to  the  refraction  of  the  cover  glass. 
Next  focus  upon  the  upper  surface  of  the  cover  glass.  The  dif- 
ference between  the  last  two  readings  will  give  the  apparent 
thickness  a  of   the  cover.     The  true  thickness  x  of  the  cover 

x 
can  now  be  found  from  the  equation  1.52  =  -  where  1.52  is  the 

a 

refractive  index  of  glass,  and  x  —  a  will  be  the  value  to  be  sub- 
tracted from  the  thickness  of  the  liquid  layer  found  above  in  order 
to  obtain  the  true  thickness.     Call  this  corrected  thickness  A. 

Now  push  the  preparation  along  so  that  only  the  object  slide 
appears  in  the  field.  Focus  sharply  upon  the  upper  surface. 
Without  disturbing  the  adjustment  push  the  slide  to  one  side  so 
that  it  is  no  longer  in  the  field  and  by  means  of  the  Abbe  con- 
denser project  the  image  of  a  net  ruled  scale  into  the  plane  of  the 
upper  surface  of  the  object  slide.  (See  page  156.)  This  is  done 
by  adjusting  the  Abbe  condenser  until  the  scale  is  very  sharp 
and  clear  in  the  microscope.  Under  no  circumstances  must  the 
focus  of  the  microscope  be  changed.  Push  the  slide  into  the 
field  so  that  an  observation  may  be  made  outside  the  cell  but 
through  both  slide  and  cover  glass,  that  is  in  the  line  Mi.  The 
image  of  the  net  rulings  will  be  faint  and  hazy.  Read  the  fine 
adjustment  and  focus  up  by  means  of  it  until  the  rulings  beconu- 
as  sharp  and  clear  as  in  the  first  place.  There  is  thus  obtained 
the  amount  of  the  displacement  of  image  due  to  slide  and  cover 
glass.     Without  changing  the  focus,  move  the  preparation  until 


198  ELEMENTARY   CHEMICAL   MICROSCOPY 

an  observation  may  be  made  through  the  filled  cell,  i.e.,  in  the 
vertical  line  M2.  The  image  of  the  net  will  now  either  be  in- 
visible or  badly  out  of  focus;  having  recorded  the  reading  on  the 
fine  adjustment,  again  focus  up  until  the  image  of  the  net  be- 
comes sharp  and  clear;  read  the  fine  adjustment.  This  value 
is  the  amount  of  displacement  due  to  slide,  cover  glass  and  cell 
contents.  The  difference  between  the  first  reading  obtained  and 
the  second  gives  the  amount  of  displacement  00'  of  the  image  O 
due  to  the  liquid  in  the  cell;  subtract  this  last  value  from  the 
depth  of  the  cell  A ;  the  remainder  8  equals  O'D .     The  refractive 

index  of  the  liquid  is  therefore  n  =  —  ■ 

Providing  great  care  is  exercised  in  the  micrometric  measure- 
ments the  determination  of  the  displacement  of  image  due  to  the 
object  slide  and  cover  glass  may  be  eliminated  as  follows:  Pro- 
ject the  image  of  the  grating  into  the  focal  plane  with  no  slide  in 
the  field,  move  the  slide  until  an  observation  can  be  made  through 
both  slide  and  cover  glass  (vertical  line  Mi),  set  the  micrometer 
of  the  fine  adjustment  at  zero  and  focus  the  plane  of  the  net  by 
means  of  the  screw  adjustment  of  the  substage  condenser;  the  dis- 
placement of  the  image  due  to  slide  and  cover  glass  has  thus  been 
eliminated.  Without  further  changing  the  focus  of  the  optical 
systems  either  above  or  below  the  stage,  move  the  cell  containing 
the  liquid  so  that  an  observation  can  be  made  through  the  center 
of  the  cell  (vertical  plane  M2).  Focus  up  with  the  fine  adjust- 
ment; the  reading  of  the  scale  will  give  the  displacement  O'O, 

.-.  8  =  A  -  O'O  and  n  =  -  • 

8 

In  all  cases  where  measurements  are  made  by  means  of  the 
fine  adjustment,  first  turn  the  graduated  head  until  the  pillar 
of  the  instrument  is  raised  sufficiently  to  allow  for  a  liberal  move- 
ment up  and  down  in  focusing.  A  number  of  readings  should 
always  be  taken  of  the  position  of  the  focal  planes  and  the  results 
averaged,  never  forgetting  to  lower  the  objective  slightly  below 
the  position  of  the  sharpest  focus  and  then  raise  it  until  the 
image  appears  most  sharply  defined,  thus  avoiding  the  error  due 
to  "back-lash." 


THE   DETERMINATION   OF   REFRACTIVE   INDFX  199 

It  is  obvious  that  the  cell  must  be  accurately  ground  in  order 
that  the  cover  glass  shall  he  parallel  to  the  object  slide,  or  if  not 
truly  parallel,  that  the  measurement  of  the  depth  of  the  cell  and 
that  of  the  displacement  of  the  image  be  made  at  the  same  point. 
Since  there  is  always  a  thin  film  of  liquid  between  the  cover  glass 
and  top  of  the  cell,  the  value  for  A  should  be  determined  with 
the  cell  rilled  and  all  data  necessary  for  the  computation  be  made 
at  once. 

This  method  gives  values  to  three  decimals  for  n  of  which  two 
places  at  least  will  be  correct  and  the  third  not  far  from  the  true 
value. 

Correct  results  are  more  easily  obtained  with  red  or  yellow 
light  than  by  ordinary  daylight. 

In  the  absence  of  a  suitable  cell,  a  simple  container  for  the 
liquid  may  be  made  from  narrow  strips  of  glass  cut  from  an  ordi- 
nary thin  object  slide  and  laid  as  shown  in 
Fig.  in.  These  strips  of  glass  are  easily  cut 
with  a  glazier's  diamond  or  with  the  sharp  end 
of  a  file. 

The  liquid  to  be  studied  is  allowed  to  drop 
into  the  opening  between  the  glass  strips,  and 
the  cell  upon  being  covered  remains  rilled  by 
capillarity.  The  cover  is  gently  pressed  down  and  the  excess 
of  liquid  removed  with  absorbent  paper  or  a  piece  of  drawn 
out  glass  tubing.  Since  there  is  a  film  of  liquid  in  this  case 
between  both  the  upper  and  lower  surfaces  of  the  cell  walls, 
considerable  care  must  be  exercised  to  avoid  serious  error.  In 
any  event  the  results  are  to  be  regarded  as  approximations 
only. 

A  number  of  other  more  accurate  methods  for  the  microscopic 
determination  of  the  refractive  indices  of  liquids  have  been 
proposed,  but  these  require  specially  constructed  prisms,  wedges 
or  lenses,  or  fragments  of  glass  of  known  index  of  refraction. 
For  information  as  to  methods,  apparatus  and  accuracy  the 
student  is  referred  to  the  excellent  paper  by  F.  E.  Wright,  The 
Measurement  of  the  Refractive  Index  of  a  Drop  of  Liquid, 
Journal  Washington  Academy  Sciences  4,  (191 4),  269. 


2CO  ELEMENTARY   CHEMICAL   MICROSCOPY 

Determining  Thickness  by  Displacement  of  Image.  —  It  is 

obvious  from  the  above  discussion  that  if  we  have  a  transparent 
body  whose  refractive  index  we  know,  we  can  determine  its 
thickness  by  applying  similar  methods.  Supposing  in  the  dia- 
gram, Fig.  no,  we  are  dealing  with  a  solid  body.  Its  thickness 
will  be  T  =  n  O'D.  In  this  case  the  value  of  n  is  known,  and 
O'D  can  quickly  be  ascertained  experimentally.  The  value  for 
T  thus  found  will  be  accurate  within  approximately  0.02  mm. 

In  the  absence  of  a  cover  glass  gauge,  the  thickness  of  cover 
glasses  or  of  object  slides  may  be  thus  determined:  place  a  tiny, 
very  thin  drop  of  ink  upon  the  upper  and  upon  the  lower  sides  of 
the  glass  plate,  so  that  they  fall  almost  in  the  same  line;  focus 
first  upon  the  lower  surface  of  the  glass,  using  the  ink  spot  as  a 
guide,  read  the  fine  adjustment  and  focus  up  until  the  upper  sur- 
face of  the  slide  is  in  focus,  again  read  the  fine  adjustment;  the 
difference  between  the  two  readings  gives  the  displacement  of 
image.  Taking  for  the  value  of  n  for  cover  glasses  and  ordinary 
object  slides  1.52,  the  thickness  is  readily  calculated  from  the 
formula  given  above. 

Glass  varies  according  to  its  composition  from  n  =  1.52  to 
n  =  1.59.     For  quartz,  n  =  1.544  to  1.553. 


THE   DETERMINATION   OF   REFRACTIVE   INDEX 


201 


Table   I. 

LIQUIDS  FOR  THE   DETERMINATION   OF  THE   REFRACTIVE 
INDICES  OF  SOLIDS   BY   IMMERSION   METHOD. 


Index  of 
refraction.1 


32 

36 
37 
39 
40 

44 
46 

46 

47 
47 
47 
48 

49 

49 

5° 

5i 

52 

55 

56 

57 

58 

58 

61 

62 

62 

625 

63 

65 

762 

953(?) 


Name. 


Methyl  alcohol 

Ethyl  ether 

Ethyl  alcohol 

Heptane 

Amyl  alcohol 

Chloroform 

Carbon  tetrachloride 

Cajeput  oil 

Glycerine 

Turpentine 

Olive  oil , 

Castor  oil 

Xylene 

Benzene 

Clove  oil 

Cedar  wood  oil 

Monochlorbenzene 

Nitrobenzene 

Monobrombenzene 

Orthotoluidine 

Monobromphenol 

Bromoform 

Quinaldin 

Monoiodobenzene 

Quinoline , 

Carbon  bisulphide 

Alpha-monochlornaphthalene 
Alpha-monobromnaphthalene 

Methylene  iodide 

Phenyl  sulphide 


Approximate 

boiling  point. 

°C. 


66 

35 
78 
98 
132 
61 
76 

174 
290 

155 


136 

80 


132 
209 

155 
197 

195 
149 
240 
187 

239 
46 

255 
277 
180 
272 


Approximate  density. 


O.79 
O.71 

0.79 
°-73 
0.83 
1.48 

1  59 
0.92 
1. 61 
0.86 
0.91 
0.96 
0.86 
0.88 
105 
0.98 
1 .04 
1 .20 
1.49 
1 .00 


83 
05 
83 
09 
29 

5° 
50 
34 


w  =  i  .36* 


tt=  I  .49* 

n  =  i-5o2 
«=i.543 


m=i-59 


n=i  .64s 


1. 12  n  =  i.8+ 


»  The  values  for  n  in  this  column  are  those  obtained  in  the  author's  laboratory  at  20°-22°  C. 
by  means  of  the  refractometer  on  Merck  products. 
2  Schroeder  van  der  Kolk,  1.  c. 
»  Kley,  1.  c. 


202 


ELEMENTARY   CHEMICAL   MICROSCOPY 


Table  II. 

LIQUIDS   FOR   DETERMINATION   OF   REFRACTIVE   INDICES 
OF   MINERALS,    CRYSTALS,    ETC. 

Wright's  Series. 

Bui.  158,  Carnegie  Institute. 


For  indices 

Use  mixtures  of 

from 

to 

1-45° 
I  .480 

I.540 
I  .640 

1.66 

1.740 
1.790 

1-475 
i-535 

1-635 
1-655 

1.740 

1.790 

1 .960 

Petroleum  and  turpentine. 

Turpentine  and  ethylene  bromide  or  turpentine 
and  clove  oil. 

Clove  oil  and  alpha-monobromnaphthalene. 

Alpha-monobromnaphthalene  and  alpha-mono- 
chlornaphthalene. 

Alpha-monobromnaphthalene  and  methylene 
iodide. 

Sulphur  dissolved  in  methylene  iodide. 

Methylene  iodide,  antimony  iodide,  arsenic  sul- 
phide, antimony  sulphide,  sulphur. 

This  series  requires  the  use  of  but  few  liquids  and  keeps  the  dispersion  of  the  liquids  within 
narrow  limits  throughout  the  series.  As  prepared  for  use,  each  one  of  the  series  should  differ 
from  the  next  above  or  below  by  0.005.  The  value  of  n  in  each  mixture  made  must  first  be  de- 
termined by  means  of  a  refractometer. 


Table  III. 
MEDIA   FOR   REFRACTIVE    INDEX    DETERMINATIONS.1 

Weighing  out  and  grinding  together  in  a  mortar  the  weights  of  the  substances  given  in  the 
table,  a  series  of  eutectics  is  obtained,  each  of  which  will  have  the  refractive  index  indicated  in 
the  first  column.     Checking  with  a  refractometer  is  unnecessary. 


Refractive 
index. 


487 

5°5 

535 

54 

55 

56 

i-57 

1.58 

i-59 
1 .60 
1.605 


Components  in  grams. 


Thymol ....  35 

Thymol 67 

Salol 60 

60 

60 

60 

60 

60 

60 

60 

60 


Alpha-naphthyl- 
amine 


5 

14 
24 
34 
44 
60 
82 
100 


1  Merwin,  J.  Wash.  Acad.  Sci.,  3  (1913),  35- 


THE   DETERMINATION   OF   REFRACTIVE   INDEX 


203 


Table  IV. 

COMPOUNDS  BELONGING  TO  THE  ISOMETRIC  SYSTEM  WHOSE 
CRYSTALS  MAY  BE  USED  FOR  THE   DETERMINATION 
OF  THE  REFRACTIVE   INDICES  OF   LIQUIDS. 


Refractive 
index.1 


439 
45° 
459 
481 

485 
490 

494 
Si5 
544 
553 
559 
566 

57i 
640 

645 
650 

657 
667 

698 

700 
755 
788 

95  + 
071 


Name. 


Sodium  alum 

Potassium  alum 

Ammonium  alum 

Potassium  chromium  alum . 

Ammonium  iron  alum 

Potassium  chloride 

Rubidium  chloride 

Sodium  chlorate 

Sodium  chloride 

Rubidium  bromide 

Potassium  bromide 

Strontium  nitrate 

Barium  nitrate 

Ammonium  chloride 

Cesium  chloride 

Rubidium  iodide 

Potassium  chlorostannate. 
Potassium  iodide 

Cesium  bromide 

Ammonium  iodide 

Arsenic  trioxide 

Cesium  iodide 

Lead  nitrate 

Silver  chloride 


Formula. 


Na2S04  •  Al 

K2S04  •  Alj 

(NH4)2S04 

K2S04  •  Cr2 

(NH4)2S04 

KC1 

RbCl 

NaCIO, 

NaCl 

RbBr 

KBr 

Sr  (N03)2 

Ba  (N03)2 

NH4C1 

CsCl 

Rbl 

K2SnCl6 

KI 

CsBr 

NHJ 

As203 

Csl 


2  (S04)3  •  24  H20 
(S04)3  •  24  H20 

•  Al,  (S04)3  ■  24  H20 
(S04)3  •  24  H20 

•  Fe2  (S04)3  •  24  H20 


Pb  (NO,),. 
AgCl 


n  lies  between 
1 .69  and  1 .71 


Bolland  gives 
Csl  n=  1 .95 
Groth  gives  1 .  782 


1  Most  of  these  values  for  n  are  taken  from   Groth 's  tables. 
Leipzig,  1906-10. 


Chemische  Krystallographie, 


204  ELEMENTARY   CHEMICAL  MICROSCOPY 

Table  V. 

REFRACTIVE  INDICES  AND  CHARACTER  OF  DOUBLE 
REFRACTION  OF  TYPICAL  CRYSTALS. 


Name. 


Ammonium  nickel  sulphate .  . . 

Ammonium  oxalate 

Ammonium  persulphate 

Barium  chloride 

Copper  sulphate 

Magnesium  sulphate 

Mercuric  chloride 

Mercuric  cyanide 

Potassium  antimonyl  tartrate . 

Potassium  arsenate 

Potassium  bichromate 

Potassium  nickel  sulphate 

Potassium  nitrate 

Potassium  persulphate 

Potassium  sulphate 

Silver  nitrate 

Sodium  borate  (tetra) 

Sodium  nitrate 

Sodium  phosphate  (tertiary) . . . 
Sodium  phosphate  (secondary) 

Sodium  thiosulphate 

Strontium  antimonyl  tartrate. 

Sucrose 

Tartaric  acid 

Urea 

Zinc  sulphate 


Formula. 


(NH4)2  Ni  (SO,),  -6K 

(NH4)2C204-  H20 

(NHJ.SA 

BaCl,  •  2  H20 

CuS04  •  5  H20 

MgSCv7H20 

HgCl2 

Hg(CN), 

K(SbO)C4H4CyH20 

HjKAsCu 

K2Cr207 

K2Ni(S04)2-6H,0 

KXO3 

K2S2O8 

K2S04 

AgN03 

Na^BA  •  10  H20 

NaN03 

Na3P04  •  12  H20 

HNa2P04-i2H20 

Na2S,03  •  5  H,0 

Sr  (SbO)2  (C4H40,)2 

Ci2H22On 

H2C4H40j 

CO(NH2)2 

ZnS04  •  7  H20 


O 


Refractive  index,1 

Crystal 

V 

system 

a  oro). 

0or«. 

7- 

M 

1.489 

1.498 

1.508 

0 

I 

438 

547 

I 

595 

M 

I 

498 

502 

I 

587 

M 

I 

63s 

646 

I 

660 

Tr 

I 

514 

536 

I 

543 

O 

I 

432 

455 

I 

461 

O 

I 

74 

71 

I 

72 

T 

I 

65 

60 

0 

I 

619 

636 

I 

637 

T 

I 

57 

52 

Tr 

I 

72 

74 

I 

82 

M 

I 

484 

492 

I 

505 

O 

I 

335 

505 

I 

506 

Tr 

1 

461 

467 

I 

566 

0 

I 

493 

494 

I 

498 

0 

I 

729 

I 

788 

M 

I 

446 

469 

I 

472 

H 

I 

58 

33 

H 

I 

44 

45 

M 

I 

432 

436 

I 

437 

M 

I 

488 

508 

I 

536 

H 

I 

638 

587 

M 

I 

538 

566 

I 

571 

M 

I 

583 

566 

I 

571 

T 

I 

485 

61 

0 

- 1 

46 

48 

I 

49 

Double 
refrac- 
tion. 


+ 

+ 
+ 
+ 


+ 

+ 

+ 
+ 
+ 


+ 

+ 


+ 
+ 


1  Values  for  n  have  been  taken  from  Groth's  tables  and  checked  in  the  laboratory.  For  uni- 
axial crystals  the  first  column  is  a>  and  the  second  t.  For  biaxial  crystals  the  first  column  is  a, 
the  second  0  and  the  third  y. 


REFERENCES. 

Tables  of  refractive  indices  in  the  following  articles  will  be  found  by  the  analyst 
of  great  value  in  the  identification  of  compounds  by  means  of  the  immersion 
method. 
Schroeder    van    der    Kolk  —  Tabellen    zur    mikroskopischen    Bestimmung   der 

Mineralien  nach  ihren  Brechnungsindex,  Zeit.  anal.  Chem.,  38  (1899),  615. 
Kley  —  Ein  Beitrag  zur  Analyse  der  Alkaloide,  Zeit.  anal.  Chem.,  43  (1904),  160. 
Bolland  —  Die  Brechnungs  indices  der  weinsauren  Alkaloide  nach  Einbettungs- 

methode.     Monats.,  29  (1908),  991.     Die  Brechnungsindices  krystallinischer- 

chemischer  Individuen  nach  der  Einbettungsmethode  von  Standpunkte  der  ana- 

lytischen  Praxis.     Monats.,  31  (1910),  387. 


CHAPTER  X. 
QUANTITATIVE  ANALYSIS  BY  MEANS  OF  THE  MICROSCOPE. 

Some  of  the  most  difficult  problems  with  which  the  chemist  has 
to  deal  are  those  requiring  an  opinion  as  to  the  probable  per- 
centage composition  or  amount  of  adulteration  in  materials  which 
cannot  be  chemically  analyzed.  As  typical  examples  of  these 
cases  may  be  cited,  mixtures  of  starches,  meals,  adulterated 
flours,  spices,  teas  and  other  food  products;  mixtures  in  which 
"firsts"  have  been  sophisticated  with  an  inferior  quality  of  the 
same  material;  adulterated  pigments;  mixtures  of  wood  pulps, 
paper  pulps,  textile  fibers,  etc. 

In  the  solution  of  problems  of  the  above  type  there  are  several 
possible  methods  of  procedure,  but  only  two  need  occupy  our 
attention.  That  these  methods  may  be  sufficiently  accurate  for 
our  purpose  the  following  requirements  must  be  met.  The 
components  of  the  mixture  must  differ  sufficiently  in  their 
appearance  under  the  microscope  to  permit  their  easy  recogni- 
tion, or  they  must  be  readily  differentiated  by  their  different 
behaviors  towards  stains  or  reagents;  the  components  must  not 
differ  materially  one  from  the  other  in  specific  gravity  and  must 
be  small  enough  in  size  to  allow  mounting  on  an  object  slide  and 
covering  with  an  object  glass;  if  of  different  specific  gravities, 
their  specific  gravities  must  be  known. 

We  may  (i)  compare  preparations  made  from  the  mixture  of 
unknown  percentage  composition  with  preparations  made  from 
similar  mixtures  of  known  percentage  composition  which  have 
been  carefully  prepared  in  the  laboratory;  or  (2)  we  may  by 
micrometric  measurements  ascertain  the  volume  of  the  com- 
ponent whose  percentage  in  the  mixture  is  sought  and  from  its 
known  density  compute  its  weight  and  hence  its  per  cent  in  the 
mixture;  or  (3)  in  the  case  of  solids  made  by  fusion  where  the 
melt  on  freezing  has  been  found  to  give  rise  to  phases  sufficiently 

205 


206  ELEMENTARY   CHEMICAL   MICROSCOPY 

characteristic  in  appearance  yet  differing  according  to  the  per- 
centage composition,  the  recognition  of  these  crystalline  phases 
will  serve  to  indicate  the  probable  composition  of  the  mass. 

The  last  method  (3)  is  restricted  to  materials  such  as  alloys 
or  related  substances.  An  expert,  knowing  the  characteristic 
appearance  following  certain  treatments,  is  able,  on  studying 
materials  of  known  components  but  of  unknown  percentage,  to 
decide  upon  the  probable  proportion  of  the  chief  constituents 
without  the  necessity  of  a  quantitative  analysis.  This  type  of 
analysis  by  means  of  the  microscope  can  be  practiced  only  by 
experts  after  long  study  and  investigation  and  cannot  therefore 
be  here  discussed. 

The  first  method  may  be  employed  in  the  quantitative  analy- 
sis of  all  mixtures  consisting  of  individual  particles,  fragments  or 
crystals,  which  are  not  too  large  for  microscopic  examination, 
providing  the  component  particles  differ  sufficiently  in  appear- 
ance to  permit  of  identification  and  that  mixtures  of  known  per- 
centage composition  can  be  prepared  in  the  laboratory.  Since 
this  method  has  its  chief  application  in  estimating  the  amount 
of  adulteration  in  a  substance,  the  discussion  will  be  confined  to 
this  aspect  only. 

Method  I.  —  Prepare  three  standard  mixtures  containing  the 
same  components  as  the  commercial  products  to  be  examined. 
In  preparing  these  standards  the  adulterant  must  be  carefully 
weighed  out  and  added  to  a  definite  weight  of  the  pure  product; 
after  thorough  mixing,  three  mixtures  of  known  per  cent  of  adul- 
teration are  thus  obtained. 

From  each  one  of  these  standards  in  turn,  several  like  portions 
are  taken,  placed  upon  glass  object  slides  in  a  drop  or  two  of 
suitable  medium  (usually  glycerine  and  water  1  :  i),1  distributed 
uniformly  in  the  mounting  medium  and  covered  with  a  square 
cover  glass,  care  being  taken  to  avoid  air  bubbles;  use  just 
sufficient  mounting  medium  to  ensure  an  even  distribution  of  the 
material  throughout  the  whole  area  covered  by  the  cover  glass 

1  Smith,  Health  Mag.,  5  (189S),  286,  has  shown  that  in  the  case  of  starch 
mixtures  a  mounting  medium  of  equal  parts  of  glycerine,  water  and  50  per  cent 
acetic  acid  is  preferable. 


QUANTITATIVE  ANALYSIS  BY  MEANS  OF  THE  MICROSCOPE         207 

and  to  completely  fill  the  space  below  the  confining  cover  yet  not 
have  a  loss  by  the  squeezing  out  of  the  liquid.  One  of  the  prepara- 
tions is  then  placed  upon  the  stage  of  the  microscope,  and  a  count 
is  made  of  the  number  of  particles  of  the  adulterant  which  are 
found  in  a  field  of  the  microscope.  Having  counted  the  foreign 
particles  in  several  different  fields,  a  second  preparation  from 
the  same  mixture  is  tried  and  so  on  until  at  least  twenty  or  more 


11 

—-/— 

10 

9 

8 

a 

0 

c3   7 

u    ' 

B 
3 
"2  6 

C3 

a 
0  5 

U 

4 

3 

2 
1 

/  1 

' 

1 

1 

I 

1 

1 

1 

1 

1 

I 

1 

I 

1 

3  i  5  6         7  8  9         10         11 

Average  number  of  foreign  particles  per  field 

Fig.  112. 


12        13 


11 


counts  have  been  made.  A  different  mixture  is  then  taken  and 
the  number  of  foreign  particles  determined  exactly  as  in  the  first. 
Finally,  the  third  known  mixture  is  examined  and  counts  made 
as  before.  Upon  a  sheet  of  "coordinate"  paper  lay  out  per 
cents  of  adulteration  as  ordinates  and  numbers  of  foreign  par- 
ticles as  abscissas.  The  averages  of  the  counts  of  these  particles 
obtained  in  each  of  the  three  mixtures  of  known  per  cent  adul- 
teration are  then  marked  upon  the  coordinate  paper  in  their 


20S 


ELEMENTARY   CHEMICAL   MICROSCOPY 


proper  places,  and  a  line  is  drawn  through  the  zero  and  the  three 
points;  the  "plot"  obtained  will  be  substantially  a  straight  line  if 
the  work  has  been  properly  done.  If  the  points  laid  out  show 
a  marked  deviation  from  a  straight  line  the  components  differ 
materially  in  their  densities,  or  an  error  has  been  made.  There 
is  thus  obtained  a  device,  Fig.  112,  by  which  we  can  determine, 
from  a  count  of  the  foreign  particles  in  any  similar  mixture,  the 
per  cent  of  this  foreign  matter  present  in  material  of  unknown 
percentage  composition.1 


:;!; 

Fig.  113. 


Fig.  114. 
Net  Ruled  Eyepiece  Micrometers. 


Fig.  115. 


To  facilitate  counting  an  eyepiece  with  net  micrometer  is 
essential.  Rulings  are  usually  of  two  types,  as  shown  in  Figs. 
113  and  114.  Where  type  113  is  employed  the  entire  field  of 
view  may  be  counted  but  in  type  114  it  is  better  to  call  a  "field" 
that  area  comprised  within  the  ruled  square.  This  system  is 
preferable  to  that  of  employing  a  cell  with  ruled  bottom  referred 
to  below.  An  attachable  mechanical  stage  will  be  found  to  be 
a  great  help  in  avoiding  the  making  of  counts  in  the  same  area 
more  than  once. 

Although  the  method  just  described  appears  at  first  sight  to 
be  crude  and  unreliable  it  has  been  found  after  a  number  of 
years'  trial  in  the  hands  of  a  large  number  of  students  to  yield 
excellent  results. 

In  the  case  of  starch  mixtures,  where  the  foreign  component 

1  Chamot,  Seventh  International  Congress  Applied  Chemistry,  Section  VIIIc 
(1909),  249. 


QUANTITATIVE  ANALYSIS  BY  MEANS  OF  THE  MICROSCOPE      209 

is  present  in  the  proportion  of  3  to  7  per  cent  the  results  found 
are  very  close  to  the  actual  per  cent,  but  when  7  per  cent  is 
reached,  the  beginner  has  trouble  in  obtaining  reliable  counts, 
and  above  10  per  cent  the  method  requires  great  manipulative 
skill. 

It  must,  however,  be  borne  in  mind  that  a  method  of  this 
sort  even  at  its  best  gives  merely  a  close  approximation  to  the 
true  value. 

The  chief  difficulties  which  will  be  encountered  are  those  of 
removing  equal  amounts  in  every  case  upon  the  end  of  a  tiny 
spatula;  of  obtaining  a  uniform  distribution  of  the  material 
throughout  the  drop;  and  of  lowering  the  cover  glass  upon  the 
preparations  without  destroying  the  uniformity  of  distribution 
of  particles  or  introducing  air  bubbles.  A  little  practice,  how- 
ever, will  enable  the  analyst  to  work  rapidly  and  accurately. 

If  more  nearly  accurate  sampling  is  desirable,  a  portion  of 
the  material  is  carefully  weighed  out,  spread  on  a  piece  of  glass 
or  glazed  paper  in  a  thin  square  of  as  nearly  uniform  thickness 
as  possible  and  then  sampled  by  "quartering"  in  the  usual 
manner1  until  a  section  equivalent  to  2  to  4  milligrams  is  obtained 
for  transfer  to  the  object  slide. 

An  even  better  method  consists  in  carefully  weighing  out  a 
small  portion  of  the  material  to  be  examined  and  mixing  it  with 
a  known  weight,  several  times  greater,  of  a  finely  and  uniformly 
powdered  substance  very  soluble  in  water  (or  other  solvent). 
After  thorough  mixing,  a  small  portion  of  the  preparation  is 
removed,  accurately  weighed  and  transferred  to  an  object  slide. 
The  selected  mounting  liquid  is  added,  causing  the  soluble 
diluting  solid  to  dissolve  and  disappear,  leaving  a  known  weight 
of  the  insoluble  material  under  investigation  evenly  spread  upon 
the  slide.  The  number  of  foreign  particles  in  this  tiny  portion 
can  then  be  counted.  In  the  case  of  most  food  products,  such 
as  starches,  flour,  meals,  spices,  etc.,  powdered  sucrose,  dextrose, 
lactose  or  soluble  dextrine  are  most  useful  as  diluents. 

When  the  mixtures  under  examination  are  of  a  density  only 
very  slightly  greater  than  water  and  are  insoluble  therein,  and 

1  Kraemer,  J.  Am.  Chem.  Soc,  21  (1899),  659. 


2IO 


ELEMENTARY   CHEMICAL   MICROSCOPY 


therefore  if  suspended  would  subside  only  after  a  long  period,  it 
is  possible  to  weigh  out  a  portion  of  the  mixture,  add  it  to  water,  or 
better,  water  and  glycerine,  in  a  small  graduated  flask,  fill  to 
the  mark,  shake  well  and  quickly  remove  one  cubic  centimeter 
or  less,  for  counting.  This  method  precludes  an  error  arising 
from  non-uniform  quantities  but  is  longer  and  more  cumbersome 
than  the  methods  already  described. 


Fig.  116.     Object  Slide  Ruled  in  One-half  Millimeter  Squares. 

Instead  of  using  a  net  ruled  micrometer  eyepiece  some  micros- 
copists  employ  a  slide  ruled  in  squares  or  a  tiny  cell  with  ruled 
bottom,  as  shown  in  Figs.  116  and  117.1  The  advantage  of  such 
devices  of  permitting  the  use  of  any  eyepiece  is  usually  outweighed 
by  a  number  of  undesirable  features,  chief  among  which  may 
be  mentioned  the  objections  that  the  rulings  on  the  slides  are 


Fig.  117.     Girard  Counting  Cell  for  the  Analysis  of  Flour. 

not  always  clear  when  the  particles  to  be  counted  are  in  focus; 
the  relatively  large  size  of  the  ruled  squares  with  a  high  power; 
and  the  difficulty  of  properly  cleaning  the  slides  without  event- 
ually injuring  the  rulings. 

When  it  is  desirable  to  cover  a  definite  area  on  the  object 
slide  it  is  far  better  to  employ  a  micrometer  disk-diaphragm 

1  Made  by  Nachet  et  Fils,  Paris,  France. 


QUANTITATIVE  ANALYSIS  BY  MEANS  OF  THE  MICROSCOPE      211 


properly  calibrated  and  inserted  into  the  eyepiece  or  to  cut  a 
square  opening  in  a  disk  of  dull  black  paper,  thin  card,  metal  or 
blackened  mica,  and  drop  the  disk  into  the  proper  eyepiece  by 
removing  the  eye-lens  and  allowing  the  disk  to  rest  upon  the 
diaphragm  of  the  eyepiece.  The  proper  size  of  opening  is  ascer- 
tained by  eyepiece  and  stage  micrometers,  and  a  square  hole  of 
this  calculated  size  is  cut  in  the  paper  and  the  perforated  disk 
is  inserted  in  the  eyepiece.  The  final  adjustment  is  then  made 
with  the  draw-tube. 

When  the  particles  of  material  are  of  a  sufficiently  low  density 
to  remain  suspended  for  a  few  seconds  and  one  cubic  centimeter 
portions  can  be  removed  the  Sedgwick-Raf  ter  counting  cell  used 


Fig.  118.     Counting  Cell.     (After  Whipple.) ' 


in  the  quantitative  determination  of  the  microscopic  organisms 
in  water  may  be  profitably  employed.  This  cell,  Fig.  118,  con- 
sists of  a  glass  object  slide  of  standard  size  to  which  is  cemented 
a  brass  cell  5  centimeters  long  by  2  centimeters  wide;  its  area  is 
therefore  1000  square  millimeters  and  being  made  exactly  1 
millimeter  deep,  its  capacity  when  closed  with  a  cover  glass  is 
1  cubic  centimeter.  Counts  of  particles  are  made  in  as  large 
a  number  of  fields  as  possible,  using  a  net  eyepiece  micrometer 

1  From  The  Microscopy  of  Drinking  Water,  by  G.  C.  Whipple,  p.  35,  Third  Ed. 
John  Wiley  and  Sons,  Inc.     Reproduced  here  through  the  courtesy  of  the  author. 


212  ELEMENTARY  CHEMICAL  MICROSCOPY 

or  an  eyepiece  with  a  central  diaphragm  opening  adjusted  to  any 
convenient  area  on  the  slide.  Results  may  be  expressed  either 
in  numbers  per  cubic  centimeter  or  in  per  cent  by  the  plotting 
method  described  above.1 

In  the  biological  examination  of  water  the  microscopic  organ- 
isms are  concentrated  into  a  few  cubic  centimeters  of  water  by  a 
small  sand  filter  contained  in  the  stem  of  a  funnel  of  special 
design.  The  sand,  together  with  the  supernatant  small  volume 
of  water,  is  emptied  into  a  test  tube,  given  a  rotary  motion  and 
as  soon  as  the  heavy  sand  subsides,  the  water  containing  the 
organisms  in  suspension  is  poured  off  and  one  cubic  centimeter 
transferred  to  the  counting  cell.2  Although  used  primarily  for 
the  purpose  stated,  this  counting  cell  and  method  can  be  applied 
to  many  problems  involving  chemical  analyses. 

In  order  to  facilitate  the  counting  and  recording  of  the  sus- 
pended matter  found  in  water,  Whipple  has  devised  an  eye- 
piece micrometer  with  special  ruling.  This  type  of  micrometer 
has  been  found  desirable  as  an  aid  in  recording  the  size  and 
number  of  masses  of  amorphous  matter  in  water.  By  common 
consent  American  analysts  have  agreed  to  express  these  values 
in  terms  of  the  areas  covered  by  the  masses  found  in  the  cell. 
The  unit  employed  is  a  square,  20  microns  on  a  side,  and  there- 
fore equal  to  400  square  microns;  this  is  known  as  a  "standard 
unit."  The  eyepiece  micrometer  is  ruled  and  so  adjusted  that 
with  a  given  objective  and  eyepiece  the  smallest  squares  are 
equal  to  a  standard  unit,  Fig.  115. 

Method  II.  —  When  isolated  particles  of  sufficiently  definite 
shape  can  be  found  and  are  of  known  composition  and  density, 
it  is  possible  to  calculate  their  weight  from  micrometric  measure- 
ments. 

1  For  further  applications  of  Method  I,  see  Meyer,  Zeit.  Nahr.  u.  Genuss,  17 
(1909)  497:  Ezendam,  Zeit.  Nahr.  u.  Genuss.,  18  (1909)  462.  Analysis  of  Starch 
Mixtures. 

Young,  Bull,  no,  Bureau  Chem.,  U.  S.  Dept.  Agric;  Pollen  in  Honey. 

Boedemann,  Landvv.  Vers.  Sta.,  75,  134;  Smut  Spores  in  Flour,  etc. 

Oerum,  Biochem.  Zeit.,  35  (1912),  18;  Fat  in  Milk:  Vauflart,  Ann.  Falsif.,  4 
(191 1)  381;  Analysis  of  Meals. 

2  For  details  and  precautions  in  water  examination,  the  student  should  consult 
Whipple,  The  Microscopy  of  Drinking  Water.     New  York,  Wiley  &  Sons,  Inc. 


QUANTITATIVE  ANALYSIS  BY  MEANS  OF  THE  MICROSCOPE      213 

This  method  is  especially  useful  in  estimating  the  weight  of 
substances  imbedded  in  other  materials  in  such  a  way  as  to  be 
not  easily  separated;  in  the  determination  of  poisons  in  forensic 
investigations;  and  in  determining  the  weight  of  tiny  metallic 
beads  or  pieces  of  metal,  which,  for  one  reason  or  another,  can- 
not be  weighed  on  a  balance. 

The  dimensions  of  the  particles  are  first  determined  by  any 
one  of  the  micrometric  methods  described  in  Chapter  VII. 
From  these  measurements  the  volumes  of  the  particles  are 
calculated  and  their  weight  then  obtained  by  multiplying  by 
the  specific  gravity  of  the  substance. 

If  the  substance  whose  weight  is  to  be  determined  can  be 
made  to  take  the  form  of  a  sphere  the  data  found  are  usually 
as  accurate  as  those  obtained  by  weighing,  but  it  is  obvious  that 
if  only  more  or  less  irregular  particles  or  crystals  are  available 
the  method  should  be  regarded  as  giving  merely  approximate 
results.  Even  so,  the  method  must  be  recognized  as  of  value 
since  in  many  instances  no  other  system  of  solving  the  problem 
of  percentage  composition  may  be  available. 

This  method  of  quantitative  analysis  by  means  of  the  micro- 
scope is  very  old  and  has  been  successfully  applied  to  the  deter- 
mination of  gold  and  silver  in  fire  assays  (especially  with  the 
blowpipe)  where  the  metallic  beads  obtained  on  cupellation 
are  too  small  to  weigh  even  upon  a  sensitive  assay  balance. 
With  carefully  fused  beads  it  has  been  shown1  that  the  results 
are  accurate  and  quickly  obtained. 

The  first  essential  is  that  the  little  metallic  globule  shall  be 
a  perfect  sphere.  If  it  is  not,  it  is  placed  in  a  tiny  cavity  in  a 
piece  of  charcoal  and  fused  before  the  blowpipe;  after  cooling, 
it  is  transferred  to  a  drop  of  glycerine  and  water  (1:1)  on  a 
glass  object  slide  by  picking  it  up  with  a  drawn-out  glass  rod 
slightly  moistened.  Bring  the  metallic  sphere  under  the  center 
of  the  micrometer  eyepiece,  use  an  objective  of  low  power, 
illuminate  with  axial  light,  with  the  Abbe  condenser  well  lowered 
using  a  small  diaphragm  opening.  Focus  up  slowly  and  as  soon 
as  the  image  reaches  its  maximum  diameter  record  the  scale 

1  Goldschmidt,  Zeit.  anal.  Chem.,  16  (1877),  434. 


214  ELEMENTARY   CHEMICAL   MICROSCOPY 

reading.  Make  several  observations  of  the  diameter  of  the 
sphere.  Then  illuminate  the  sphere  by  oblique  light  by  swinging 
the  mirror  far  to  one  side;  determine  the  diameter  again,  making 
not  less  than  three  observations;  the  results  should  be  the  same 
as  the  measurements  made  with  axial  light.  Average  the  results. 
The  weight  of  the  bead  may  now  be  calculated  from  the  equa- 
tion W  =  {dz  X  0.5236)  5  where  d  is  the  diameter  of  the  sphere 
and  s  the  specific  gravity  of  the  metal.1 

For  the  quantitative  determination  of  minute  particles  of  mer- 
cury micrometric  measurements  of  the  diameters  of  the  globules 
of  the  metal  and  calculations  of  weight  therefrom  are  also  un- 
questionably one  of  the  oldest  and  best  methods  at  our  disposal 
in  toxicological  examinations,  in  the  analysis  of  mineral  waters, 
urine,  gases  carrying  mercury  vapors,  etc. 

Raaschou2  has  recently  worked  out  in  great  detail  the  methods 
and  conditions  essential  for  the  quantitative  separation  of 
minute  amounts  of  mercury  from  liquids.  For  details,  the 
student  should  consult  the  original  article.3  When  dealing 
with  sublimates  of  metallic  mercury  consisting  of  so  great 
a  number  of  tiny  globules  as  to  render  measurements  of  the 
diameters  of  all  the  globules  impracticable,  cause  them  to 
unite  into  a  few  large  spheres  by  stirring  the  film  with  a  fine 
needle,  or  stiff  hair,  or  glass  rod  drawn  down  to  a  hair,  but  if 
this  is  done  the  needle  or  hair  must  always  be  examined  with 
the  microscope  to  see  that  no  mercury  has  been  removed  by 
clinging  to  the  stirrer.  In  order  that  accurate  measurements 
may  be  made  it  is  essential  that  the  globules  of  metallic  mercury 
shall  never  be  so  large  that  they  become  flattened  and  thus  not 
perfect  spheres.  In  determining  the  diameter  of  the  spheres 
proceed  exactly  as  described  above,  always  making  several 
measurements  of  the  sphere  diameters.  From  the  average  of 
the  data  thus  obtained,  calculate  the  weight  W  =  (d3  X  0.5236) 
X  13-59- 

1  For  gold,  s  =  19.33;    silver  =  10.4;    platinum  =  21.15;   lead  =  ir-36;    mer- 
cury =  13.59. 

2  Raaschou,  Zeit.  anal.  Chem.,  49  (1910),  172. 

3  See  also  page  319,  Microchemical  Detection  of  Mercury. 


QUANTITATIVE  ANALYSIS  BY  MEANS  OF  THE  MICROSCOPE      215 

In  estimating  the  percentage  of  the  different  fibers  entering 
into  the  composition  of  a  given  sample  of  paper,  it  is  customary 
in  most  commercial  paper-testing  laboratories  to  guess  at  the 
per  cent  of  a  given  fiber  without  comparison  with  standards  and 
without  counting  the  fibers,  the  usual  practice  being  for  several 
analysts  to  "guess"  at  the  composition  independently.  These 
men  in  time  become  very  expert  and  their  findings  will  generally 
check  within  one  per  cent.  In  the  opinion  of  the  author,  com- 
paring with  known  standards,  using  the  comparison  microscope 
or  comparison  eyepiece  is  quicker  and  gives  a  more  reliable 
approximation. 

Herzog1  has  suggested  a  microscopic  method  for  the  quan- 
titative estimation  of  the  different  fibers  in  fabrics,  or  for 
the  per  cent  of  different  colored  fibers  in  a  fabric.  Stated 
briefly,  the  process  is  as  follows:  A  tiny  piece  of  the  fabric  is 
imbedded  in  paraffin  (M.P.  6o°)  by  repeated  dipping.  After 
cooling,  sections  about  0.1  to  0.2  millimeter  are  cut  by  means  of 
a  razor  or  microtome  knife.  One  of  the  sections  is  transferred 
to  an  object  slide,  warmed  until  the  paraffin  melts  and  is  tipped 
back  and  forth  to  evenly  distribute  the  fiber  fragments.  A 
drop  of  balsam  is  placed  upon  a  cover  glass  and  lowered  upon 
the  preparation.  The  entire  number  of  each  different  fiber  is 
then  ascertained  by  counting,  using  a  net  eyepiece  micrometer. 
Having  thus  found  the  relative  proportion  of  the  fibers,  their 
absolute  size  is  next  determined  by  measurements  of  length 
and  thickness,  or  since  the  thickness  of  the  section  cut  is 
known  and  the  average  diameter  of  different  fibers  is  also  well 
established,  actual  dimensional  measurements  may  not  be  re- 
quired. The  weight  is  calculated  by  multiplying  the  absolute 
size  by  the  number  of  fragments  and  the  specific  gravity  of 
the  fiber. 

Quantitative  microchemical  methods  with  reference  to  the 
handling  of  minute  amounts  of  material  and  weighing  on  a 
Nernst  micro  balance;  the  titration  of  tiny  volumes  of  liquid; 
the  measurement  of  tiny  volumes  of  gas,  etc.,  which  do  not  re- 
quire the  application  of  the  microscope  need  no  discussion  hero, 
1  Herzog,  Z.  Chcm.  Ind.  Kol.,  I  (1907),  202.     Z.  Text.  Ind.,  1006,  No.  4. 


2l6  ELEMENTARY   CHEMICAL  MICROSCOPY 

since  we  are  dealing  solely  with  the  application  of  the  micro- 
scope to  the  solution  of  chemical  problems.1 

Volume  and  Weight  Per  Cents  from  Area  Measurements.  — 
The  quantitative  analysis  of  heterogeneous  material  in  thin  sec- 
tions through  the  determination  of  the  areas  occupied  by  the 
different  components,  as  ascertained  from  their  images  when 
seen  in  the  microscope,  has  long  been  employed  by  petrologists. 

The  process  is  briefly  as  follows:  The  outlines  of  the  areas  of 
the  component  under  consideration,  in  a  given  field  of  the  micro- 
scope, are  traced  upon  coordinate  paper  by  means  of  a  drawing 
camera;  the  value  of  a  square  of  the  paper  is  ascertained  with 
a  stage  micrometer  as  hereinbefore  described.  The  areas  of  the 
tracing  may  then  be  computed  or  may  be  accurately  determined 
by  means  of  a  planimeter.  Or  the  preparation  may  be  photo- 
graphed with  a  coordinate  (net-ruled)  ocular  in  place,  the  value 
of  the  rulings  in  the  image  ascertained  in  the  usual  manner  and 
the  areas  of  the  different  component-sections  in  the  photograph 
computed.2 

From  the  computed  areas,  volume  per  cents  may  be  calcu- 
lated, and  knowing  the  specific  gravities  of  the  components, 
weight  per  cents  are  easily  ascertained. 

This  method  of  quantitative  microscopic  analysis  has  recently 
been  applied  by  Johnson  to  the  examination  of  concretes.  He 
has  shown 3  that  it  is  a  simple  matter  to  ascertain,  whether,  in  a 
given  concrete  structure,  a  contractor  has  complied  with  the 
specifications  as  to  proportions  of  sand,  gravel  and  cement 
and  further  whether  the  material  was  properly  mixed  and  wetted. 

Estimation  of  Molecular  Weights  by  Micrometric  Measure- 
ments. —  Barger 4  has  described  a  most  ingenious  micrometric 
method  whereby  the  molecular  weight  of  a  substance  may  be 
determined,  providing  a  large  enough  amount  of  the  material 
for  weighing  upon  an  analytical  balance  is  available. 

A  solution  is  made  of  known  weight  content  of  the  substance 

1  See  Donau,  Die  Arbeitsmethoden  der  Mikrochemie,  Stuttgart,  1913. 

2  For  further  details  as  to  rock  analysis  and  for  bibliography  see  Johannsen, 
Petrographic  Methods,  p.  290. 

8  Eng.  Record,  Mar.  1915. 

4  Barger.     J.  Chem.  Soc.  (London),  85  (1904),  286. 


QUANTITATIVE  ANALYSIS  BY  MEANS  OF  THE  MICROSCOPE     217 

whose  molecular  weight  is  sought.  A  second  solution  of  known 
strength  is  also  made  of  a  substance  of  known  molecular  weight. 
Drops  of  these  two  solutions  are  introduced  alternately  into  a 
thin-walled  capillary  tube  having  a  bore  whose  diameter  is 
from  1  to  2  millimeters.  The  tube  should  be  6  to  8  centimeters 
long.  Between  the  drops  which  occupy  a  space  about  1  to  3 
millimeters  long  there  must  be  air  spaces  equal  to  approximately 
twice  the  lengths  of  the  drops.  The  first  and  last  drops  should 
be  those  of  the  standard  and  from  two  to  three  times  the  length 
of  the  intermediate  drops.  After  the  drops  are  in  place  the 
capillary  tube  is  sealed  at  both  ends.  The  tube  is  then  laid 
upon  an  object  slide  and  cemented  in  place  with  Canada  balsam 
or  other  suitable  medium,  the  slide  is  then  immersed  in  water 
in  a  suitable  shallow  vessel  and  placed  under  the  microscope. 
By  means  of  a  micrometer  the  lengths  of  the  drops  are  deter- 
mined and  recorded  in  scale  divisions  but  not  in  absolute  units. 
After  standing  for  about  an  hour  measurements  are  again  made. 
Owing  to  differences  in  vapor  pressure,  some  drops  have  increased 
in  length;  others  have  decreased. 

The  theory  of  the  method  is  thus  described  by  its  author: 
"Each  drop  is  placed  between  two  others  of  a  different  solution, 
and  can  evaporate  on  either  side  into  a  small  air-chamber. 
This  chamber  is  soon  saturated  with  vapor,  which  can  condense 
freely  on  the  drops.  If  the  vapor  pressures  of  the  two  solutions 
are  equal  the  evaporation  will  equal  the  condensation,  and  there 
will  be  no  change  in  volume  of  the  drops.  If,  on  the  other 
hand,  the  vapor  pressures  are  unequal,  there  will  be  a  gradient 
of  vapor  pressure  in  the  air  spaces;  some  drops  will  therefore 
be  in  contact  with  an  atmosphere,  the  vapor  pressure  of  which  is 
greater  than  their  own.  Condensation  will  take  place  on  these 
drops  and  they  will  increase.  The  others,  alternating  with 
them,  will  have  a  vapor  pressure  greater  than  that  of  the  ad- 
joining air  spaces;  these  drops  will  evaporate  and  thus  decrease. 
Hence,  there  is  a  distillation  from  the  drops  of  one  series  to 
those  of  the  other  series.  By  measurement  we  can  tell  which 
drops  increase  and  hence  ascertain  which  solution  has  the 
smaller  vapor  pressure.     If  the  solvent  is  identical  in  both  cases 


2l8 


ELEMENTARY   CHEMICAL   MICROSCOPY 


and  if  the  solutes  are  non-volatile,  the  solution  with  the  smaller 
vapor  pressure  will  have  the  greater  concentration  of  molecules 
and  vice  versa. " 

A  series  of  tubes  must  be  made  in  which  the  strength  of  the 
standard  solution  has  been  systematically  varied  in  small  frac- 
tions of  a  gram-molecule  per  liter.  A  tube  is  thus  obtained  in 
the  series  where  there  is  little  variation  in  the  lengths  of  the 
drops  of  known  and  unknown  or  where  there  is  change  in  the 
character  of  the  variation,  say  from  an  increase  in  length  to  a 
decrease  in  length.  It  is  evident  that  the  molecular  concentra- 
tion of  the  unknown  must  correspond  to  that  of  the  known  solu- 
tion at  this  point. 

Weight  of  unknown  in  grams  per  liter 
Concentration  in  gram-molecules  found 

This  may  be  made  clear  by  quoting  one  experiment:  Standard 
used,  cane  sugar.     Unknown,  glucose.  .  Solvent,  water. 


Molecular  weight 


Concentration  of 

Standard 

in  gram-molecules. 

Nature  of  change 

in  length  of  drop 

of  unknown. 

Tube  i 

Tube  2 

Tube  3 

0.05 
O.  10 
O.I2 

OI3 
O.  14 

OI5 

O.  20 

O.25 

Increase 

*  1 

i  ( 
Decrease 

1  1 
1  1 
(  t 

Tube  4 

Tube  5 

Tube  6 

Tube  7 

Tube  8 

It  is  evident  therefore  that  the  concentration  of  the  unknown 
material  lies  between  the  concentrations  of  tubes  number  4  and 
5,  that  is  between  0.13  and  0.14  gram-molecule  per  liter.     Hence, 


molecular  weight  = 


2^.02  2s. 02 

=  179,  or  — —  =  192. 


That  is,  the 


0.14  0.13 

molecular  weight  of  the  unknown  lies  between  179  and  192;  the 
average  =  185.5.     Calculated  for  glucose,  C6Hi206  =  180. 

It  appears  from  a  very  large  number  of  experiments  that  this 
method  is  a  simple  and  dependable  one,  apparently  subject  to 
errors  no  greater  than  those  usually  inherent  in  macroscopic 
molecular  weight  determinations. 


QUANTITATIVE  ANALYSIS  BY  MEANS  OF  THE  MICROSCOPE      2ig 

As  small  amounts  as  25  to  50  milligrams  may  successfully  be 
used. 

For  special  precautions,  sources  of  error  and  suggestions  as 
to  the  choice  of  solvents  and  standards,  the  student  is  referred 
to  the  original  article. 

This  method  of  Barger's  for  the  determination  of  molecular 
weights  is  another  example  of  the  manifold  applications  of  the 
microscope.  The  microscopist  whose  laboratory  is  seldom 
equipped  with  apparatus  for  the  determination  of  molecular 
weights  by  the  usual  methods  of  boiling  or  freezing  points,  or 
by  vapor  densities,  may  nevertheless  obtain  sufficiently  accurate 
results  for  all  practical  purposes  by  the  procedure  outlined 
above. 

The  method  is  worthy  of  far  more  attention  by  analysts 
than  it  has  been  given. 


CHAPTER  XI. 

THE  DETERMINATION  OF  THE  MELTING  AND  SUBLIMING 
POINTS  OF  MINUTE  PARTICLES  OF  MATERIAL. 

The  determination  of  the  melting  point  of  a  compound  is 
usually  one  of  the  simplest  and  most  reliable  tests  at  our  dis- 
posal for  ascertaining  the  purity  of  a  known  compound  or  for 
obtaining  an  idea  as  to  the  probable  nature  of  a  substance  of 
unknown  composition.  In  the  case  of  organic  compounds  the 
melting  point  is  one  of  the  first  constants  to  be  ascertained  and 
even  with  certain  inorganic  substances  a  melting  point  deter- 
mination may  often  prove  of  great  value. 

It  not  infrequently  happens  that  such  a  small  quantity  of 
material  is  available  that  the  usual  laboratory  methods  are  im- 
practicable and  recourse  must  be  had  to  some  microscopic 
method  of  procedure.  Often,  the  chemist  deals  with  material 
containing  a  large  proportion  of  amorphous  matter  mixed  with  a 
crystalline  substance  and  a  satisfactory  separation  cannot  be 
effected;  or  again,  a  preparation  is  obtained  in  which  there 
appears  to  be  two  or  more  different  crystalline  substances  but 
no  means  for  separating  them  can  be  found.  In  all  these  cases 
a  melting  point  would  give  the  needed  information  were  it  possible 
to  effect  a  separation. 

By  spreading  out  the  material  in  a  thin  layer  upon  an  object 
slide  and  examining  the  preparation  with  the  microscope,  we  can 
almost  always  find  crystals  or  fragments  of  material  here  and 
there  not  in  direct  contact  with  others,  but  appearing  in  the 
image  isolated  and  free.  We  have  thus  in  reality  affected  a 
separation  and  if  we  apply  heat,  we  should  be  able  to  make 
reliable  observations  upon  the  behavior  of  each  isolated  particle. 
If  in  addition  we  have  some  means  of  controlling  and  measuring 
the  heat  applied,  it  is  obvious  that  a  melting  point  can  be  ascer- 
tained.    Inasmuch  as  a  variety  of  methods  for   temperature 

220 


DETERMINATION   OF   MELTING   POINTS  221 

measurements  are  available,  it  follows  that  melting-point  deter- 
minations may  be  obtained  of  material  actually  invisible  to  the 
naked  eye.  Furthermore,  these  determinations  will,  in  most 
cases,  be  as  accurate  as  those  made  by  the  usual  capillary-tube 
sulphuric-acid  method. 

Method  A.  (Approximate.)  —  Where  a  series  of  pure  com- 
pounds, readily  crystallizable  and  each  of  known  melting  point 
is  at  hand  the  melting  point  of  an  unknown  substance  may  be 
ascertained  approximately  by  placing  similar  sized  fragments 
of  the  known  and  the  unknown  side  by  side  at  the  corner  of  a 
thin  object  slide.  The  rotating  stage  of  the  microscope  is  re- 
moved and  a  piece  of  asbestos  board,  perforated  at  the  center, 
substituted  as  a  stage.  A  bent  glass  or  quartz  tube  drawn  out 
to  a  jet  at  one  end  serves  as  a  tiny  burner  and  may  be  fastened 
temporarily  to  the  substage  ring.  The  tiny  burner  is  so  adjusted 
that  the  flame  falls  nearly  in  the  line  of  the  optic  axis  of  the 
microscope.  The  slide  carrying  the  material  to  be  tested  is 
placed  under  the  microscope  and  focused  and  the  tiny  flame  is 
very  slowly  brought  nearer  the  preparation  by  means  of  the 
screw  which  serves  to  raise  or  lower  the  substage.  The  behavior 
of  the  material  is  watched  very  closely  through  the  microscope, 
to  determine  whether  the  known  or  the  unknown  substance 
melts  first.  Other  compounds  of  known  melting  point  are  tried 
until  a  known  compound  is  found  with  which  the  unknown 
simultaneously  melts  or  the  unknown  is  found  to  melt  between 
the  melting  points  of  two  knowns.  This  indirect  method  is 
quick  and  convenient  where  mere  approximations  are  needed. 
The  operator  after  one  or  two  trials  soon  learns  to  judge  the 
temperatures  given  by  the  tiny  burner  according  to  the  size  of 
its  flame  and  the  distance  below  the  slide.  When  comparing 
melting  points  in  this  manner  first  try  the  pure  material  with 
which  the  unknown  is  believed  to  be  identical.  Place  the  two 
substances  so  close  together  on  the  slide  that  when  they  melt, 
the  molten  masses  will  flow  together;  if  they  melt  simultaneously 
and  mix  to  form  a  homogeneous  melt,  the  presumption  is  strong 
that  the  two  fragments  are  of  the  same  composition.  If  so,  when 
the   melt   solidifies    (freezes)    a   single   component   will    result. 


222 


ELEMENTARY    CHEMICAL   MICROSCOPY 


Lehmann1  long  ago  pointed  out  that  this  method  of  "fusion 
testing"  could  be  made  use  of  in  qualitative  analysis  but  the 
interpretation  of  the  phenomena  which  may  be  observed,  usu- 
ally requires  a  profound  knowledge  of  chemistry  and  much 
practice  in  manipulation. 

In  the  Appendix  will  be  found  a  table  giving  the  melting  points 
of  compounds  which  can  be  employed  in  making  estimations  of 
melting  points  by  the  process  described  above. 

Method  B.  (Exact.)  —  Melting  points  below  the  boiling  point 
of  water  may  be  determined  with  great  accuracy  by  means  of 
a  hot  stage  through  which  hot  water  is  made  to  circulate.  A 
convenient  form  of  apparatus  is  shown  in  Fig.  119.2    It  consists 


Fig.   119.     Apparatus  for  the  Determination  of  Low  Melting  Points. 


of  a  glass  box  or  trough,  such  as  is  commonly  employed  for  the 
spectroscopic  examination  of  liquids,  the  open  end  of  which  is 
provided  with  a  wedge-shaped  piece  of  rubber,  forming  a  tight 
stopper.  The  hot  water  enters  the  cell  through  the  glass  tube  A 
and  escapes  at  B,  the  rate  of  flow  being  controlled  by  a  stop- 
cock or  screw-clamp.  The  hot  water  may  conveniently  be  ob- 
tained by  siphoning  it  through  a  small  coil  of  copper  pipe  D 
heated  by  a  Bunsen  burner  E.  Or  the  heating  system  devised 
for  providing  a  continuous  flow  of  hot  water  through  a  Zeiss 

1  O.  Lehmann,  Die  Krystallanalyse,  Leipzig,  1891. 

2  Chamot  and  Albrech,  Unpublished  paper  presented  to  the  Cornell  Section, 
Am.  Chem.  Soc;  May,  1906. 


DETERMINATION  OF   MELTING   POINTS 


223 


butyrorefractometer  may  be  employed.  By  regulating  the 
heating  flame  and  the  rate  of  flow  of  hot  water,  very  gradual  or 
very  rapid  rises  of  temperature  may  be  obtained  or  the  temper- 
ature may  be  maintained  almost  constant.  Jacketing  the  cell 
with  asbestos  simplifies  the  regulation  of  temperature.  Heaters 
functioning  on  the  principle  of  the  thermo-siphon,  Fig 
also  be  employed  for  temperatures  up 
to  85  to  900  C;  but  above  90  de- 
grees the  regulation  of  the  height  of 
the  heating  flame  becomes  rather  diffi- 
cult and  the  sudden  formation  of 
steam  usually  results  in  a  blow-off 
through  the  safety  tube,  in  which  the 
thermometer  is  only  very  closely  in-  - 
serted. 

Substituting  brine  or  oil  for  water, 
the  temperatures  can  be  raised  to 
125-150  degrees  if  the  heating  coil  be 
used,  but  the  author  has  never  found 
hot  oil  to  give  satisfactory  results  in 
any  thermo-siphon  system,  since  the  Fig. 
viscosity  of  the  oil  in  the  glass  cell 
is  too  great  to  permit  an  even  and  sufficiently  rapid  rate  of  flow 
unless  large  conducting  pipes  be  employed,  necessitating  a  cell 
far  too  thick  for  use. 

The  temperatures  are  most  conveniently  measured  by  means 
of  a  set  of  Anschiitz  thermometers.  Thermometers  of  this 
type  are  sufficiently  small,  so  as  not  to  project  too  far,  and  their 
graduations  are  such  as  to  permit  readings  to  be  taken  to  0.1 
degree. 

A  convenient  arrangement  for  reading  the  thermometer  and 
observing  the  melting  point  of  the  substance  under  observation 
is  given  below. 

With  hot  stages  of  the  sort  just  described  it  is  always  a  wise 
precaution  to  place  the  cell  in  a  glass  tray  or  shallow  crystallizing 
dish  to  guard  .against  damage  to  the  microscope  should  the  hot 
stage  break. 


120.     Heater  for  Melting 
Point  Apparatus. 


224 


ELEMENTARY   CHEMICAL   MICROSCOPY 


Any  flat  surfaced,  stoppered  container  may  serve  as  a  hot 
stage,  as,  for  example,  a  small  flat  bottle. 

For  temperatures  above  1500  C.  the  only  convenient  and  uni- 
versally applicable  heating  system  is  by  means  of  an  electric 
current,  resistance  wire  and  suitable  rheostat.  The  heating 
coil  in  this  case  may  consist  of  manganin,  nichrome  or  platinum 
wire.  To  obtain  the  best  and  most  reliable  results  part  of  the 
heating  coil  should  be  above  the  object  being  heated  and  part 
below. 

Fig.  121  shows  an  electrically  heated  hot  stage  which  has 
been  in  use  in  the  author's  laboratory  for  several  years.     It 


CS> 


pbi 

|9f     i 

w 

i    A    i 

;        1 
1        jl 

c  i 

Fig.  121.     Apparatus  for  the  Determination  of  Melting  and  Subliming  Points. 


consists  of  a  low  cylinder  of  "Alberene  stone"  closed  at  the  top 
and  bottom  by  thin  glass,  or  by  mica  when  high  temperatures 
are  employed.  The  heating  coil  H,  H  consists  of  fine  platinum 
wire  wound  in  fine  coils.  In  the  illustration  A  shows  the  Alberene 
stone;  B,  brass  guides  for  the  object  slide  acting  as  cover;  C, 
adjustable  wire  fingers  for  supporting  cover  glasses,  tiny  crucibles, 
"micro"  retorts,  etc.;  D  is  a  removable,  thin  brass  diaphragm 
cutting  down  the  opening  of  the  stage  and  serving  as  a  radiator; 
T,  thermometer;  PP,  binding  posts;  M,  mica  or  glass  window 
closing  the  bottom  of  the  hot  stage;  and  S,  the  object  slide  cover. 
The  method  of  inserting  the  hot  stage  for  use  in  place  of  the  rotat- 
ing stage  is  shown  in  Fig.  122.  By  attaching  an  Abbe  camera 
lucida  to  the  microscope  tube  and  properly  tipping  the  mirror, 
the  image  of  the  scale  of  the  thermometer  may  be  so  reflected 
as  to  be  seen  alongside  of  the  material  whose  melting  point  is 


DETERMINATION   OF   MELTING  POINTS 


22: 


Fig.  122.     Polarizing  Microscope  arranged  for  Observing  Melting  Points. 


226  ELEMENTARY   CHEMICAL  MICROSCOPY 

to  be  determined.  A  lens  attached  to  the  body-tube  or  held  in 
a  separate  stand  serves  to  magnify  the  thermometer  scale.  It 
is  thus  possible  to  look  into  the  tube  of  the  instrument  and  to 
watch  both  the  material  and  the  thermometer.  This  arrange- 
ment and  its  applications  will  be  readily  understood  by  refer- 
ence to  the  illustration. 

With  platinum  wire  coils  a  temperature  somewhat  higher  than 
7000  C.  may  be  obtained  in  the  apparatus. 

The  material  to  be  tested  may  be  either  crystallized  upon  or 
supported  on  a  small  thin  cover  glass  held  by  the  wire  fingers  C 
or  may  be  placed  in  a  short  piece,  5  millimeters  long,  of  tiny  thin- 
walled  capillary  tube  fastened  to  the  thermometer  by  a  wire 
band.  For  ordinary  materials  these  tubes  are  best  held  horizon- 
tally but  for  fats,  waxes,  etc.,  better  results  are  obtained  by 
slightly  inclining  the  capillary  and  taking  as  the  melting  point 
the  thermometer  reading  at  the  instant  the  fat  slides  out  of 
focus. 

The  melting  point  of  anisotropic  substances  is  sharply  obtained 
by  making  the  observations  with  crossed  nicols  and  a  selenite 
plate;  the  change  from  solid  to  liquid  of  tiny  particles  is  thus 
remarkably  clear  since  they  vanish  instantly  on  melting.  The 
hot  stage  should  in  such  cases  be  provided  with  glass  windows. 

The  upper  window  of  the  stage  consists  of  a  thin  glass  object 
slip  (or  one  of  mica  or  of  quartz)  held  in  place  by  the  guides  B,  B, 
permitting  sliding  the  cover.  This  is  essential  when  dealing  with 
materials  which  sublime,  for  in  these  cases  the  upper  window 
becomes  fogged  with  condensed  material,  and  in  such  an  event 
the  cover  is  simply  pushed  along  until  a  clear  section  is 
obtained. 

In  determining  melting  points  with  any  type  of  hot  stage,  it  is 
obvious  that  the  usual  procedure  should  be  followed,  namely: 
make  a  preliminary  observation  and  then  start  anew,  raising  the 
temperature  very  gradually  as  the  melting  point  first  observed 
is  approached. 

Determinations  of  the  subliming  points  of  tiny  particles  may 
also  be  made  by  means  of  the  hot  stage. 

Electrically  heated  stages  of  several  forms  and  for  different 


DETERMINATION  OF  MELTING  POINTS  227 

ranges  of  temperature  may  now  be  had  from  several  different 
optical  firms.1 

The  Determination  of  Subliming  Points  may  be  made  in  the 
hot  stage  illustrated  in  Figs.  121  and  122,  or  by  the  crucible 
method  of  Blyth  described  on  page  243. 

1  E.  Leitz,  Wetzlar,  Germany,  manufactures  some  especially  convenient  hot 
stages.  For  other  types  of  hot  stage  see  Cram,  J.  Am.  Chem.  Soc,  34  (191 2),  954; 
and  Cottrell,  J.  Am.  Chem.  Soc,  34  (191 2),  1328. 


CHAPTER  XII. 

METHODS  FOR  THE  HANDLING  OF  SMALL  AMOUNTS  OF 

MATERIAL. 

Microchemical  Methods.  —  By  microchemical  methods  we 
generally  mean  the  application  of  chemical  operations  to  the 
examination  and  study  of  very  small  quantities  of  material.  The 
chief  chemical  operations  with  which  we  have  to  deal  are:  i. 
Solution;  2.  Decantation;  3.  Filtration;  4.  Sublimation;  5. 
Distillation;  6.  Precipitation;  7.  Ignition,  Fusion,  and  Mis- 
cellaneous Treatments. 

Since  success  in  chemical  microscopy  requires  skill  in  the 
technique  of  these  operations  each  one  will  be  discussed  at 
length. 

1.  Solution.  Testing  for  Solubility. -- At  the  corner  of  a 
perfectly  clean  object  slide  of  glass,  quartz,  or  celluloid,  place  a 
small  drop  of  water  (or  other  solvent);  the  drop  should  be  3  to  4 
millimeters  in  diameter  and  about  1  millimeter  deep.  Place 
close  to  this  drop  a  tiny  fragment  of  the  material  whose  solu- 
bility is  to  be  tested.  Transfer  the  glass  slip  to  the  stage  of  the 
microscope  and  focus  with  a  low  power  objective  upon  the  edge 
of  the  drop  nearest  the  fragment.  See  that  the  illumination, 
using  an  Abbe  condenser,  is  carefully  adjusted,  and  that  the  iris 
•diaphragm  is  at  least  two-thirds  closed.  By  means  of  a  glass 
rod  drawn  out  line,  a  platinum  wire  or  a  stiff  hair,  slowly  push 
the  fragment  into  the  drop,  at  the  same  time  looking  into  the 
instrument  so  as  to  be  able  to  note  the  phenomena  which  may 
take  place  the  instant  the  material  enters  the  solvent;  for  ex- 
ample, the  substance  may  merely  "melt"  away,  or  it  may  de- 
crepitate, or  give  off  bubbles  of  gas,  or  it  may  dissolve  with 
decomposition  (hydrolise),  etc.  A  little  practice  is  often  neces- 
sary to  enable  the  beginner  to  push  substances  into  drops  of 
solvent  while   looking   into    the    instrument.     It   is  of    course 

228 


METHODS  FOR  HANDLING  SMALL  AMOUNTS  OF  MATERIAL         229 

necessary  to  remember  that  directions  are  reversed  in  the  image 
formed  by  the  microscope  and  seen  by  the  worker,  but  if  this  is 
borne  in  mind  there  will  soon  be  no  difficulty  in  moving  and 
turning  objects  while  observing  them  through  the  microscope. 

If,  after  a  few  minutes,  there  appears  to  be  no  change  in  the 
appearance  or  size  of  the  material  being  tested,  warm  the  drop 
gently  by  holding  it  a  second  or  two  about  one  centimeter  above 
the  "reserve"  or  "pilot"  flame  of  the  laboratory  burner  (see 
Fig.  73,  page  127).  This  tiny  flame  should  be  so  regulated  by 
means  of  the  set  screw  as  not  to  be  over  5  millimeters  high.  Cool 
the  preparation  quickly  by  holding  the  slip  for  an  instant  in  con- 
tact with  a  smooth  metal  block  placed  for  this  purpose  near  the 
burner,  or,  in  the  absence  of  such  a  cooling  device,  place  the  slide 
on  the  base  of  the  microscope.  Examine  the  fragment  of  material 
to  be  tested  and  note  any  change  in  its  appearance  and  size. 

To  heat  a  solution  to  boiling  have  a  large  drop  at  the  very 
corner  of  a  glass  slip,  tip  the  slip  slightly  so  that  the  drop  flows 
toward  the  corner  and  hold  it  so  that  the  tip  of  the  micro- 
flame  (pilot  flame)  touches  the  glass  just  below  the  upper  edge 
of  the  inclined  drop.  Watch  closely  and  as  soon  as  bubbles  rise, 
remove  from  the  flame  and  cool  instantly  by  bringing  in  contact 
with  a  cool  metal  surface.  It  is  necessary  to  work  quickly, 
otherwise  the  evaporation  will  be  so  great  that  the  preparation 
will  become  dry.  Never  place  a  hot  slide  on  the  stage  of  the  mi- 
croscope, for  the  stage  may  be  seriously  damaged  and  the  vapors 
arising  will  condense  upon  the  objectives  injuring  them.  Since 
the  drop  has  been  placed  at  the  corner  of  the  slide  there  is  no 
danger  of  the  glass  cracking  or  breaking  on  heating,  an  accident 
that  will  almost  invariably  happen  if  the  glass  slip  is  heated  at 
any  other  point  than  a  corner.  If  quartz  or  platinum  slips  are 
used,  heating  at  the  corner  is  not  essential  to  prevent  breakage, 
but  is  more  convenient. 

To  determine  whether  any  material  has  passed  into  solution, 
decant  the  liquid  from  the  undissolved  material  (see  Decanting 
below),  and  evaporate  to  dryness  very  carefully.  In  evapora- 
ting drops  to  dryness,  never  keep  the  material  over  the  flame 
until  all  the  liquid  has  been  driven  off.     Simply  warm  the  prepa- 


230  ELEMENTARY   CHEMICAL   MICROSCOPY 

ration,  then  remove  it  from  the  flame  and  blow  gently  upon  the 
warm  drop,  heat  again  and  again  blow;  repeat  the  process  until 
the  solvent  has  been  driven  off.  If  this  method  is  followed,  a 
uniform,  closely  adhering  film  will  result  instead  of  irregularly 
distributed  loose  particles,  and  the  danger  of  loss  through  de- 
crepitation of  the  tiny  solid  particles  is  avoided. 

It  is  essential  to  remember  that  it  is  impossible  to  obtain  slips 
made  of  sufficiently  resistant  glass  upon  which  water  will  not 
exert  a  marked  solvent  action ;  moreover,  it  must  constantly  be 
borne  in  mind  that  all  liquids  soon  take  up  foreign  matter  from 
the  bottles  in  which  they  are  kept.  The  results  of  tests  for 
solubility  should  always  be  checked  by  comparison  with  the 
residues  left  when  the  solvent  alone  is  evaporated  under  exactly 
the  same  conditions. 

It  follows  therefore  that  tests  for  the  solubility  of  substances 
in  boiling  liquids  or  in  strong  acids,  alkalies,  etc.,  should  be  per- 
formed on  clean,  bright  platinum  foil;  the  solvent  is  decanted, 
concentrated  and  only  transferred  to  a  glass  or  quartz  slip  when 
evaporated  almost  to  dryness. 

Should  the  illuminating  gas  be  of  very  poor  quality  and  the 
heating  prolonged,  an  amount  of  various  ammoniacal,  sulphur 
and  other  products  may  be  absorbed  by  the  solvent  sufficient  to 
vitiate  the  results. 

If  the  substance  whose  solubility  is  being  tested  is  subse- 
quently to  be  analyzed,  a  sufficient  quantity  of  it  is  tested  on 
glass,  quartz  or  platinum,  according  to  the  necessities  of  the  case, 
care  being  taken  to  observe  the  precautions  given  above  as  to 
impurities  in  solvents  and  the  probability  of  their  action  on  the 
microscopic  slides  used.  This  action  may  not  always  be  due  to 
the  solvents  alone,  but  may  be  the  result  of  the  material  being 
tested.  When  more  than  one  solvent  has  been  found,  the  choice 
will,  of  course,  be  governed  by  many  circumstances.  It  is 
obvious  that  no  fixed  rule  may  be  given  which  will  apply  to  even 
a  majority  of  cases.  Much  must  always  be  left  to  the  judgment 
of  the  analyst. 

Decantation.  For  most  purposes,  it  is  generally  possible  to 
obtain  sufficiently  clear  solutions  from  drops  containing  precipi- 


METHODS  FOR  HANDLING  SMALL  AMOUNTS  OF  MATERIAL        231 

tates  or  fragments  by  drawing  off  the  supernatant  liquid,  without 
being  obliged  to  resort  to  the  longer  and  more  tedious  methods 
of  filtration.  Success  in  drawing  off  a  liquid  requires,  in  the 
first  place,  a  perfectly  clean  slide  free  from  grease,  otherwise  the 
liquid  will  not  flow  properly;  and,  secondly,  patience,  care  and 
a  steady  hand.  The  first  requirement  is  met  by  treating  the 
slides  in  one  of  the  usual  cleaning  mixtures  of  which  the  chromic- 
sulphuric  acid  is  the  best,  and  subsequently  thoroughly  washing 
them.  Sometimes  placing  a  drop  or  two  of  ammonium  hydroxide 
on  the  slide  and  wiping  it  dry  with  a  clean  cloth  will  materially 
improve  the  surface.  The  other  requisites  for  successful  decan- 
tation  are  dependent  upon  the  manipulative  ability  of  the  analyst 
and  may  be  acquired  only  by  practice. 

Although  the  phrase  synonymous  with  decantation  -  -  draw- 
ing-off  —  is  self-explanatory  and  the  method  is  quite  obvious, 
there  are,  nevertheless,  several  points  upon  which  the  success  of 
the  operation  depends. 

Assuming  that  the  drop  of  liquid  is  situated,  as  usual,  at  the 
corner  of  the  slide,  the  operator  proceeds  as  follows:   The  slide  is 
held  in  a  horizontal  position;  the  end  of  a  drawn-out  glass  rod  or 
a  platinum  wire  is  carefully  introduced  into  the  edge  of    the 
drop  and  is  then  slowly  drawn  across  the  slide  (the  slide  being 
simultaneously  slightly  inclined  in  the  same  direction)  until  a 
distance  of  about  one  centimeter  is  reached.     If  the  slide  is  per- 
fectly clean  the  liquid  will  follow  the  rod  or  wire  in  a  narrow 
stream.    A  circular  motion  is  now  given  the  rod,  resulting  in  the 
spreading  out  of  the  little  stream  into  a  drop ;  this  induces  a  flow 
of  the  liquid  from  the  original  drop.     The  steps  in  the  decanta- 
tion are  indicated  in  Fig.  123.     The  flow  is  aided  by  increasing 
the  angle  of  inclination  of  the  slide,  providing,  of  course,  there  is  no 
tendency  on  the  part  of  the  sediment  to  flow  with  the  liquid.    The 
important  points,  which  can  be  learned  only  by  practice,  are  the 
proper  angle  and  the  rate  and  manner  of  spreading  out  the  drop. 
Should  there  be  any  tendency  of  the  sediment  to  pass  over  with 
the  liquid,  reduce  the  angle  at  once.     If  the  sediment  tends  to 
form  a  dam  and  prevent  the  passage  of  the  clear  liquid,  it  is  neces- 
sary to  start  a  new  current  at  one  side  of  the  barrier  or  to  break 


232 


ELEMENTARY   CHEMICAL   MICROSCOPY 


the  latter  down  at  a  suitable  point.  As  soon  as  the  proper 
volume  of  liquid  has  been  drawn  off,  still  holding  the  slide  in- 
clined, a  piece  of  filter  or  folded  lens  paper  is  drawn  through  the 
channel,  between  the  two  drops  at  C,  Fig.  123,  and  the  prepara- 
tion immediately  heated  gently  over  the  micro-flame  at  this 
same  point.  The  result  of  this  heating  is  the  separation  of  the 
two  drops  by  a  dry  space ;  thus  there  is  no  danger  of  the  decanted 
liquid  flowing  back  when  the  slide  is  again  placed  in  a  horizontal 
position. 


Fig.   123.     Decanting  a  Drop  of  Liquid  from  a  Precipitate. 

When  the  clear  decanted  liquid  is  not  wanted  for  analysis  and 
only  the  sediment,  or  precipitate,  in  the  original  drop  is  to  be 
utilized,  the  decanted  portion  and  connecting  stream  are  both 
wiped  off  the  slide  with  iilter  paper  while  the  slide  is  inclined  and 
the  preparation  heated  gently  below  the  wiped-off  drop  to  pre- 
vent any  farther  spreading. 

In  cases  where  the  sediment  in  the  drop  persists  in  flowing  with 
the  liquid  being  drawn  off,  and  where  heating  is  not  objection- 
able, the  slide  is  tipped  so  as  to  cause  all  the  liquid  to  again  flow 
back  into  the  original  source  and  the  drop  is  evaporated  to  dry- 
ness at  a  low  temperature,  exceptional  care  being  taken  to  pre- 
vent heating  the  residue  after  evaporation.  This  step  will 
usually  cause  the  sediment  to  cling  to  the  glass  and  to  aggluti- 
nate. A  drop  of  water  or  the  proper  liquid  is  then  carefully 
added,  the  preparation  allowed  to  stand  a  few  seconds  to  permit 
the  soluble  compounds  to  pass  into  solution  and  the  solution 
then  decanted  as  above  described.  Usually  a  clear  liquid  may 
now  be  obtained  without  difficulty. 


METHODS  FOR  HANDLING  SMALL  AMOUNTS  OF  MATERIAL        233 

Liquids  which  have  been  decanted  but  which  are  not  suffi- 
ciently clear  may  be  evaporated  and  treated  by  the  method 
described  in  the  preceding  paragraph. 

Washing  precipitates  by  decantation  may  be  performed  by 
drawing  off  the  liquid  as  above,  adding  a  drop  of  washing  liquid 
to  the  residue,  allowing  to  stand  for  a  few  seconds  and  drawing 
off  as  before.  The  process  is  repeated  as  long  as  is  thought 
necessary,  or  until  tests  applied  to  the  decanted  liquid  prove  that 
the  washing  is  sufficiently  complete.  It  is  obvious  that  with  a 
pure  solvent,  containing  no  compounds  in  solution,  the  simplest 
test  is  evaporation  to  dryness  and  the  obtaining  of  no  perceptible 
residue. 

In  the  event  of  a  number  of  drops  being  obtained  in  the  process 
of  washing,  all  of  which  must  be  saved  and  united  for  subsequent 
examination,  it  is  best  to  transfer  them  to  a  second  clean  slide; 
this  is  done  by  decanting  into  the  extreme  corner  of  the  slide, 
cutting  off  the  stream  with  filter  paper  and  warming  as  already 
described.  Now  slowly  raise  the  slide  to  an  almost  vertical 
position  and  bring  the  corner,  holding  the  decanted  drop,  in 
contact  with  the  slide  prepared  to  receive  it.  Touch  the  drop 
at  the  corner  with  a  drawn-out  glass  rod  or  platinum  wire  and 
the  drop  will  flow  at  once  on  to  the  slide  below.  Raise  the  verti- 
cally held  slide  and  warm  its  corner  over  the  micro-flame,  wash 
the  residue  as  before  and  again  transfer.  The  united  washings 
may  afterward  be  concentrated  to  the  proper  volume  by  evapo- 
ration. 

In  all  cases  where  decantation  is  to  be  practiced  the  size  of  the 
drop  to  be  treated  must  be  somewhat  larger  than  that  employed 
in  tests  alone. 

Decantation  by  Means  of  the  Centrifuge.  -  -  Next  in  impor- 
tance to  the  methods  above  described  for  separating  sediment 
from  liquid  must  be  placed  the  centrifugal  machine. 

A  "two-speed"  machine,  with  hematokrit  frame,  should  be 
purchased,1  since  it  is  seldom  that  sufficient  liquid  is  available 
in  ordinary  microchemical  work  to  permit  of  the  usual  sedimen- 
tation tubes  being  employed.     With  the  hematokrit  attachment, 

1  A  convenient  form  of  machine  is  shown  in  Fig.  76. 


234  ELEMENTARY   CHEMICAL   MICROSCOPY 

however,  very  small  quantities  of  liquids  can  be  handled,  and  the 
high  speed  obtainable  will  throw  out  even  a  precipitate  whose 
specific  gravity  differs  but  little  from  the  liquid  in  which  it  is 
suspended. 

A  convenient  form  of  tube  for  use  at  high  speeds  may  be  made 
as  follows:  An  ordinary  glass  tube  of  proper  size  is  drawn  out 
to  a  point  in  the  flame  of  the  blast  lamp,  and  then,  by  continued 
heating,  the  glass  is  allowed  to  thicken  a  little  at  the  end;  the 
end  is  pressed,  while  still  soft,  against  a  piece  of  asbestos  board, 
or  a  piece  of  charcoal,  to  flatten  it  sufficiently  to  lit  well  in  the 
hematokrit  frame.  The  tube  is  then  cut  the  proper  length,  and 
the  upper  end  smoothed  with  a  file  or  rounded  in  the  lamp  flame. 
The  turbid  liquid  to  be  treated  is  introduced  into  the  tube  by 
means  of  a  pipette  with  long  capillary  end,  and  the  tube  is  then 
placed  in  the  frame;  a  similar  tube  is  filled  with  water  to  the 
same  height,  and  is  placed  in  the  other  side  as  a  balance.  Thus 
arranged,  the  machine  is  turned  at  such  speed  and  for  such  a 
time  as  may  be  necessary  to  yield  a  clear  liquid. 

The  treatment  to  which  the  sedimentation  tube  is  then  sub- 
jected will  depend  upon  whether  the  liquid  or  the  sediment  (or 
both)  is  wanted.  When  the  clear  supernatant  liquid  is  required, 
it  is  removed  by  means  of  a  pipette  with  long  capillary  tip.  But 
when  the  precipitate  alone  is  needed  the  clear  liquid  is  most  con- 
veniently removed  by  capillary  tubes,  made  by  drawing  out  odds 
and  ends  of  glass  tubing.  With  such  tubes  it  is  only  necessary 
to  touch  the  liquid,  which  will  immediately  be  drawn  up  by 
capillarity;  the  tubes  filled  as  far  as  the  force  will  raise  the  liquid 
are  thrown  away.  One  tube  after  another  is  inserted  until  the 
liquid  is  lowered  to  a  point  just  above  the  sediment.  Distilled 
water  is  introduced,  and  if  the  precipitate  is  to  be  washed,  the 
contents  of  the  tube  are  mixed  well  with  a  platinum  wire,  and  the 
tube  is  again  whirled  to  effect  a  separation;  for  most  purposes 
one  washing  is  sufficient.  The  wash  water  is  removed  as  before, 
and  if  the  amount  of  sediment  is  very  small,  the  tube  is  cut  off 
just  above  it  to  enable  easy  removal  of  the  solid  material.  The 
upper  part  of  the  tube  is  not  wasted,  but  serves  to  make  capillary 
tubes.     These  small  sedimentation  tubes  are  easily  and  quickly 


METHODS  FOR  HANDLING  SMALL  AMOUNTS  OF  MATERIAL         235 

made.  A  stock  should  be  provided  so  that  a  number  are  always 
on  hand.  It  will  be  found  convenient  to  have  sedimenta- 
tion tubes  of  different  diameters,  to  permit  varying  amounts 
of  liquid  being  used.  Similarly  constructed  smaller  tubes  of 
thinner  wall  can  be  made  to  fit  inside  the  ordinary  "sputum" 
tubes  usually  furnished  with  the  centrifuge. 

Once  having  become  accustomed  to  using  this  instrument,  the 
worker  in  microchemistry  will  find  that  the  two-speed  centrifuge 
is  an  almost  indispensable  instrument,  which  will  enable  him  to 
meet  with  ease  all  sorts  of  problems  involving  the  separation  of 
solids  and  liquids  that  would  otherwise  tax  his  patience  and 
ingenuity. 

Especially  to  be  recommended  are  electrically  driven  centri- 
fuges provided  with  protecting  hoods. 

When  dealing  with  relatively  large  volumes  of  liquid  the  usual 
conical  sedimentation  tubes,  shown  in  Fig.  76,  will  prove  useful, 
but  since  it  is  usually  the  sediment  which  is  to  be  subjected  to 
examination  or  analysis,  and  rarely  the  liquid,  it  will  be  found 
more  convenient  to  employ  tubes  drawn  down  to  a  fairly  long 
pointed  end  which  may  be  cut  off  with  a  file  scratch  just  above 
the  sediment,  thus  permitting  easy  access  to  the  solids  thrown 
out  from  suspension.  When  properly  drawn  down,  tubes  of  this 
form  can  be  used  several  times  by  simply  sealing  the  end;  the 
tubes  are  centered  and  held  in  the  aluminum  carriers  by  means 
of  perforated  corks. 

Occasionally  tubes  with  removable  parts  will  be  found  to  be 
convenient;  the  best  forms  are  those  devised  by  T.  W.  Richards1 
for  the  separation  of  small  quantities  of  crystals  from  mother 
liquor.  The  construction  and  method  of  employment  of  these 
tubes  will  be  readily  understood  by  reference  to  Fig.  124. 

When  one  of  the  modern  large  electric  laboratory  centrifugal 
machines  2  is  available  very  minute  amounts  of  suspended  matter 
may  be  separated  from  large  volumes  of  liquid  with  great  case. 
The  most  convenient  form  of  apparatus  for  this  purpose  con- 
sists in  fitting  a  Squibb's  separatory  funnel  with  a  stopcock  of 

1  Richards,  J.  Amer.  Chem.  Soc,  27  (1905)  104. 

2  As  for  example  the  Bausch  and  Lomb  Precision  Centrifuge. 


236 


ELEMENTARY   CHEMICAL  MICROSCOPY 


the  type  provided  in  a  Spaeth  sedimentation  glass,  as  shown  in 
Fig.  125.  Upon  being  whirled  in  the  machine  the  suspended 
matter  is  forced  into  the  conical  cavity  in  the  stopcock;  a  quarter 
turn  of  the  stopcock  completely  cuts  off  the  sediment  from  the 


Wire 


Fig. 


124.     Richards  Tubes  for  Centrifugal 
Separations. 


Fig.  125.  Sedimentation  Fun- 
nel for  Large  Centrifugal 
Machines. 


liquid  and  the  latter  can  be  poured  off  without  danger  of  disturb- 
ing the  sediment;  the  stopcock  can  then  be  removed,  and  the 
contents  of  the  cavity,  containing  only  a  very  small  volume  of 
the  solution  and  all  the  suspended  matter  originally  present, 
subjected  to  examination  and  analysis. 

Filtration.  —  In  spite  of  every  precaution  it  frequently  happens 
that  decantation  will  not  yield  a  sufficiently  clear  liquid  for  sub- 
sequent reactions,  or  that  the  precipitate  cannot  be  freed  of  the 
mother  liquor,  and  that  centrifugal  separation  cannot  be  used. 
Under  such  circumstances  recourse  must  be  had  to  filtration, 
which  is  doubtless  one  of  the  most  troublesome  processes  of 
microchemical  work.  Since,  in  the  majority  of  cases,  the  amount 
of  liquid  to  be  filtered  consists  of  two  or  three  small  drops,  often 
less,  methods  involving  the  use  of  a  funnel,  be  it  ever  so  small, 


METHODS  FOR  HANDLING  SMALL  AMOUNTS  OF  MATERIAL        237 

are  to  be  regarded  as-  unsatisfactory.  In  this  category  must  be 
placed  the  ingenious  filtering  device  of  Haushofer,1  for  it  is  too 
cumbersome,  complicated,  requires  too  much  time,  and  necessi- 
tates the  transferring  of  the  solution  from  the  slide  to  the  filtering 
apparatus,  and  back  again  to  a  slide. 

There  are  at  present  several  practical  and  convenient  methods 
for  filtering  small  volumes  of  liquid,  all  based  upon  drawing  the 
liquid  through  a  tiny  bit  of  filter  paper  held  at  one  end  by  a  glass 
tube  of  small  or  capillary  bore  while  suction  is  applied  at  the 
other.  The  fundamental  differences  He  chiefly  in  the  manner  of 
applying  the  filtering  material.  None  of  these  are  to  be  recom- 
mended for  qualitative  analyses. 

The  simplest,  quickest  and  most  useful  method  is  that  of 
Behrens.2  A  filtering  tube  is  prepared,  Fig.  126,  consisting  of  a 
glass  tube  F  about  60  millimeters  long,  and  of  1.5  to  2  millimeters 
bore,  with  walls  about  1  millimeter  thick.  One  end  is  ground 
smooth  and  exactly  at  right  angles  to  the  axis;  the  other  end  is 
rounded  so  as  to  permit  the  easy  attachment  of  a  small  piece  of 
rubber  tube  R,  about  80  millimeters  long,  carrying  a  piece  of 
glass  tube  M  for  a  mouthpiece. 

The  preparation  of  the  filter  and  the  operation  of  filtering  a 
liquid  is  performed  as  follows:  A  square  piece  of  thick  soft  filter 
paper  P  of  close  texture  is  cut  slightly  larger  than  the  diameter 
of  the  tube,  and  is  placed  on  the  slide  S  (which  lies  horizontally 
on  the  table)  close  to  the  drop  D  to  be  filtered ;  the  ground  cud  of 
the  tube  is  pressed  firmly  against  the  filter  paper  near  one  edge; 
the  whole  is  then  moved  slowly  into  the  drop;  as  soon  as  the 
paper  is  wet,  gentle  suction  is  applied  to  the  upper  end  of  the  tube 
by  the  mouth,  through  the  agency  of  the  rubber  tube.  At  the 
same  time  the  filter  paper  is  slowly  advanced  still  further  into 
the  drop,  the  precipitate  unless  exceedingly  fine  will  be  pushed 
along  in  a  ridge  before  the  advancing  paper  and  the  liquid  will 
rise  in  the  tube.  Care  must  now  be  taken  to  keep  the  rubber 
tube  slightly  curved,  as  shown  in  the  cut.  As  soon  as  sufficient 
liquid  has  risen  into  the  glass  tube,  suction  is  discontinued,  the 

1  Haushofer,  Mikroskopische  Reactioncn,  Braunschweig,  1885,  p.  160. 

2  Behrens,  Anleitung  Mikrochem.  Anal.,  p.  22. 


238 


ELEMENTARY   CHEMICAL  MICROSCOPY 


rubber  tube  compressed  at  its  upper  end  between  the  fingers  and 
is  simultaneously  straightened  to  prevent  the  forcing  out  of  the 
liquid.  To  lift  the  tube  from  the  slide  and  the  piece  of  filter 
paper,  stretch  the  rubber  tube  very  gently  and  raise  the  whole 
apparatus.  The  filtrate  contained  in  the  tube  is  removed  by 
bringing  the  ground  end  in  contact  with  a  slide  and  bending  the 
rubber  tube,  the  upper  end  of  which  is  kept  closed;  the  liquid 
will  generally  flow  out  at  once;  if  not,  straighten  the  tube,  open 
the  upper  end  and  blow  very  gently,  but  only  just  sufficiently  to 
expel  the  drops. 


Fig.  126.     Behrens  Method  of  Filtration. 

A  little  practice  is  required  in  order  to  apply  the  proper  pressure 
of  the  glass  tube  upon  the  filter  paper  and  to  maintain  this 
pressure  uniformly  without  tipping  the  tube  out  of  its  vertical 
position. 

The  chief  difficulties  encountered  in  rapid  work  are:  (1)  The 
danger  of  carrying  the  filtrate  up  into  the  mouth  or  into  the 
rubber  tube  by  air  bubbles,  which  are  always  drawn  into  the  tube 
when  the  liquid  to  be  filtered  has  all  been  absorbed  by  the  filter 
paper  and  sucked  into  the  tube,  and  (2),  it  not  infrequently 
happens  that  the  filtered  liquid  begins  to  flow  out  when  suction 


METHODS  FOR  HANDLING  SMALL  AMOUNTS  OF  MATERIAL        239 

is  stopped  and  before  there  is  time  to  prevent  it  by  closing  the 
upper  end  of  the  tube.  These  difficulties  may  be  overcome  by  a 
modification  of  the  simple  filtering  tube,1  consisting  of  the  intro- 
duction of  an  inner  tube  or  trap.  A  glass  tube  about  3!  milli- 
meters internal  diameter  has  fused  into  its  vertical  axis  a  tiny 
tube  about  1  millimeter  in  diameter  and  7  to  8  millimeters  long. 
The  lower  end  of  the  main  tube  is  caused  to  flow  together  until 
the  central  opening  is  about  2  millimeters  in  diameter,  and  it  is 
then  ground  so  as  to  give  a  perfectly  flat  surface.  The  apparatus, 
which  is  30  millimeters  long,  is  attached  to  a  rubber  tube  and  is 
employed  in  the  same  manner  as  the  previously  described  filter- 
tube.  It  is  obvious  that  as  the  filtrate  rises  in  the  tube  it  over- 
flows into  the  small  trap  and  is  held  in  the  space  between  the 
walls  of  the  outer  and  inner  tubes.  The  tube  through  which 
the  liquid  rises  is  therefore  free,  and  any  air  bubbles  entering 
cannot  cause  a  loss  of  the  filtrate,  nor  can  the  liquid  flow  back 
if  suction  is  stopped.  The  filtrate  can  be  removed  either  by 
means  of  a  drawn-out  pipette  or  by  inverting  the  tube  and  in- 
ducing the  liquid  to  flow  by  means  of  a  platinum  wire. 

Savage  2  introduces  the  filter  paper  within  the  tube,  making 
the  manipulation  somewhat  simpler,  the  filtering  of  liquids  from 
very  fine  precipitates  somewhat  easier  and  permits  of  han- 
dling larger  volumes  of  liquid.  But  this  method  fails  to  handle 
as  tiny  quantities  of  liquid  as  that  of  Behrens  and  the  residue 
is  not  so  readily  separated  from  the  filter.  Savage  describes 
his  method  as  follows: 

"A  glass  tube  of  about  4  millimeters  inside  diameter  is  drawn 
out  as  abrupt  as  possible,  and  the  narrow  portion  of  the  tube 
should  extend  from  15  to  30  millimeters  from  this  point,  with 
parallel  sides  and  an  inside  diameter  of  about  eight-tenths  of  a 
millimeter.  The  entire  tube  is  8  or  9  centimeters  long,  and  both 
ends  are  rounded  in  the  lamp  flame.  From  a  piece  of  soft  filter 
paper  of  smooth  surface  and  long  fiber  a  triangular  piece  is  torn 
(not  cut),  2  to  2\  centimeters  long  and  1  centimeter  wide  at 
the  base.     This  is  rolled  between  the  fingers  into  a  slightly  taper- 

1  Chamot,  Jour.  Appl.  Micros.,  3,  S54. 

2  Savage,  Jour.  App.  Micros.,  3  (1900),  678. 


240  ELEMENTARY  CHEMICAL  MICROSCOPY 

ing,  cigar-shaped  plug.  It  should  be  rolled  dry  and  rolled  long 
enough  to  make  it  tine  and  even.  If  the  paper  is  cut,  not  torn, 
there  will  be  a  seam  in  it,  and  it  cannot  be  so  readily  made  tight. 
The  plug  thus  formed  is  inserted  in  the  small  end  of  the  tube 
from  the  outside  and  worked  in  by  rotating  the  tube  until  from  4 
to  8  millimeters  of  the  paper  are  within.  The  rest  of  the  paper  is 
then  cut  off  a  millimeter  or  two  from  the  end  of  the  tube." 

The  filter  is  first  moistened  with  distilled  water  and  then  in- 
serted in  the  drop  to  be  filtered,  suction  is  applied  to  the  larger 
end  and  the  clear  liquid  drawn  up  through  the  filter  into  the  tube, 
from  which  it  is  removed  by  a  capillary  pipette  or  by  carefully 
removing  the  filter  paper  with  a  pair  of  fine  forceps  and  expelling 
the  liquid  in  exactly  the  same  manner  as  in  the  Behrens  method. 

A  tightly  rolled  cigar-shaped  plug  of  filter  paper  or  fibrous 
asbestos  may  be  inserted  in  a  straight  Behrens  tube  in  a  similar 
manner  to  that  described  above,  and  will  be  found  to  yield  even 
more  satisfactory  results  than  the  fragile  drawn-out  tube  of 
Savage. 

The  author  has  found  in  certain  instances  that  alundum  filters 
have  proved  of  great  value.  Such  filters  are  made  by  grinding 
tiny  conical  plugs  from  pieces  of  broken  alundum  crucibles  and 
fusing  these  plugs  into  the  ends  of  glass  tubes  2  to  2.5  millimeters 
in  diameter  and  50  to  60  millimeters  long.  After  fusing,  excep- 
tional care  must  be  taken  in  cooling  and  annealing.  In  like  man- 
ner porous  porcelain  plugs  may  be  used,  but  in  such  an  event 
a  powerful  suction  pump  is  required,  suction  by  means  of  the 
mouth  being  insufficient  to  cause  the  passage  of  the  liquid. 

Sublimation.  —  This  operation,  though  of  somewhat  limited 
application  and  comparatively  seldom  employed  in  inorganic 
qualitative  analysis,  is  so  very  important,  and  of  such  inestimable 
value  in  the  examination  of  organic  compounds,  that  every  worker 
should  become  thoroughly  familiar  with  it,  particularly  with  the 
method  of  performing  fractional  sublimations. 

The  usual  method  is  that  of  sublimation  from  one  slide  to 
another.  The  material  to  be  tested  is  placed  at  the  corner  of  a 
thin  slide.  If  it  is  a  solid  it  is  wise  to  moisten  it  with  water  and 
then  dry  it  thoroughly;    this  will  generally  effectually  prevent 


METHODS  FOR  HANDLING  SMALL  AMOUNTS  OF  MATERIAL        241 

the  material  from  being  blown  off  by  air  currents,  and  brings  the 
substance  in  intimate  contact  with  the  glass  slide  —  a  matter  of 
prime  importance.  If  the  material  is  already  in  solution,  evapo- 
rate a  tiny  drop,  but  in  this  case  it  should  not  be  spread  out,  as 
is  commonly  done  with  test  drops.  When  the  drop  is  dry,  add 
another  tiny  drop  on  top  of  the  residue  left  by  the  first;  this  in 
turn  is  dried,  the  process  being  repeated  until,  in  the  judgment 
of  the  operator,  there  is  sufficient  material  for  work.  In  all 
cases  the  residue  to  be  treated  should  occupy  but  little  space, 
yet  should  not  be  too  thick,  since,  if  fractional  sublimation  is 
to  be  practiced,  a  thick  mass  is  apt  to  be  heated  unequally  and 
fallacious  results  will  be  obtained. 

Everything  being  ready,  the  slide  is  held  in  the  left  hand  and 
the  heating  begun  over  the  micro-flame,  not  directly  beneath 
the  spot  of  material,  but  slightly 

nearer    the   center  of    the    slide.  V~~~ . 

This  is  done  in  order  to  avoid  rais-  \  \ 

ing  the  temperature  too  rapidly    > V L 

and  too  high.     As  soon  as  the  sub-    \  \  \\ 

limation  point  is  almost  reached      \  \  \\ 

(which  can  easily  be  recognized       \  \  \\ 

by  practice)  a  second  clean  slide,         \  \  >^\ 

carrying  a  drop  or  two  of  water,  \ ^ ^  J\ 

is  taken  in  the  right  hand  and  (ST^-^ 

lowered  over  the  first  slip,  with 
the  drop  of  water  on  the  upper 
side    directly   over    the   material 

to    be   sublimed.      The    drop   of 

.  ,       Fig.   127.     Sublimation  of   Material 

water     has     for     its      object     the      from  One  Object  Slide  to  Another, 
keeping  of   the  upper  slide  cool, 

thus  far  more  effectually  condensing  any  vapors  produced  by  the 
heating.  The  receiving  slide  is  supported  on  an  edge  of  the  other 
and  is  brought  to  within  2  to  4  millimeters  of  the  substance  (see 
diagram,  Fig.  127).  The  temperature  is  gradually  raised  by 
moving  the  spot  of  substance  nearer  the  flame.  As  soon  as  there 
is  evidence  of  the  appearance  of  a  sublimate,  raise  the  two  slides 
above  the  flame  so  as  to  prevent  too  rapid  vaporization.     The 


242  ELEMENTARY   CHEMICAL   MICROSCOPY 

first  deposit  being  obtained,  the  receiving  slide  is  moved  along  a 
few  millimeters  and  a  second  sublimation  made;  again  the  slides 
are  partly  removed  from  the  source  of  heat,  the  receiving  slide 
moved  along  a  trifle,  and  again  the  temperature  is  raised  until  a 
third  film  has  been  condensed.  The  process  is  continued  as 
long  as  the  material  holds  out  on  the  first  slide  or  fails  to  yield 
any  further  sublimate.  If  the  drops  of  water,  used  to  keep  the 
receiver  cool,  evaporate,  replace  them  by  others.  When  dealing 
with  compounds  which  melt  on  heating,  the  supporting  slide  must 
be  slightly  inclined  so  as  to  keep  the  material  at  the  corner  of  the 
slide.  Or  we  may  sublime  from  a  watch  glass  upon  an  object 
slide,  as  shown  in  Fig.  129,  page  245. 

It  sometimes  happens  that  a  more  crystalline  and  characteristic 
sublimation  film  is  to  be  obtained  when  the  receiving  slide  is 
slightly  warm,  in  which  event  the  water  is  omitted,  or,  if  this  is 
not  sufficient,  a  little  cylinder  made  of  carbon,  such  as  is  used  in 
arc  lamps,  is  warmed  over  a  burner  and  placed  upon  the  slide. 
Such  pieces  of  carbon  remain  warm  for  some  time  and  will  be 
found  to  give  excellent  results. 

With  the  beginner  it  is  always  best  to  obtain  each  fractional 
sublimate  upon  a  separate  slide,  carefully  laying  them  down  film 
side  up  in  the  order  in  which  they  have  been  obtained.  Other- 
wise the  films  first  formed  are  apt  to  be  driven  off  by  the  in- 
creasing heat  required  to  vaporize  the  last  portions  or  will  be 
rubbed  off  by  the  fingers  or  by  contact  with  the  support. 

When  a  series  of  sublimation  films  are  obtained  upon  a  single 
slide  always  see  that  the  films  succeed  each  other  in  such  a  man- 
ner as  to  bring  the  first  ones  farther  and  farther  from  the  source 
of  heat  as  each  film  in  turn  is  formed. 

When  dealing  with  sublimations  taking  place  only  at  tempera- 
tures so  high  that  ordinary  glass  will  soften,  quartz  slips  may  be 
employed  or  nickel  or  platinum  foil  or  small  nickel  or  platinum 
spatulas.  The  method  of  procedure  will  in  any  event  be  similar 
to  that  above  described,  intimate  contact  between  substance  and 
support  being  first  accomplished  when  possible  by  moistening 
with  water  and  careful  drying. 

The  temperatures  of  sublimation  may  be  determined  by  means 


METHODS  FOR  HANDLING  SMALL  AMOUNTS  OF  MATERIAL        243 

of  a  hot  stage  such  as  that  described  on  page  224  or  by  the  method 
recommended  by  A.  W.  Blyth.1  A  small  porcelain  crucible  is 
nearly  rilled  with  mercury,  into  which  dips  the  bulb  of  a  ther- 
mometer. A  thin  cover  glass,  bearing  at  its  center  the  material 
to  be  tested,  moistened  and  dried  as  usual,  is 
floated  on  the  surface  of  the  mercury.  Upon 
the  cover  glass  is  placed  a  low  glass  cell  whose 
upper  and  lower  rims  are  accurately  ground. 
A  second  cover  glass  is  placed  above  to  receive 
the  film  —  see  diagram,  Fig.  128.  A  number 
of  clean  covers  should  be  placed  near  at  hand. 
The  crucible  is  heated  over  the  low  flame  of 
a  Bunsen  burner.  As  the  temperature  rises, 
the  covers  are  changed,  by  means  of  a  pair  of 
forceps,  every  five  or  ten  degrees.  The  cover 
glasses  are  examined  under  the  microscope, 
and  a  decision  made  as  to  the  temperature  of  FlG-  I28-  Crucible 
sublimation.     A  second  and  even  a  third  ex-         f' °   .° 

sublimation. 

periment    should  always   be   made.      If  the 

material  fails  to  sublime  at  a  temperature  below  that  at  which 

the  mercury  itself  is  volatilized,  a  bath  of  a  suitable  low-melting 

alloy  must  be  used.     For  accurate  measurements  it  is  essential 

to  protect  the  crucible  and  cell  from  the  cooling  effects  of  air 

currents. 

Subliming  upon  a  glass  object  slide  as  shown  in  Fig.  127  is 
impracticable  when  only  a  minute  quantity  of  the  material  is 
available  since  the  losses  through  incomplete  condensation  are 
considerable.  In  such  an  event  it  is  safer  to  employ  the  device 
shown  in  Fig.  130,  page  245,  primarily  intended  for  distillation 
but  yielding  good  results  with  solids  as  well  as  with  liquids. 
When,  however,  only  an  excessively  small  amount  of  material  is 
to  be  tested  as  in  toxicological  analysis,  it  is  better  to  drop  the 
substance  into  a  thin-walled  glass  tube  of  not  over  1  millimeter 
in  diameter,  sealed  at  one  end.  Tap  the  tube  gently  so  as  to  col- 
lect all  of  the  material  at  the  sealed  end.  With  a  very  line  blast- 
lamp  flame  draw  out  the  open  end  to  a  hair-like  capillary  tube, 
1  Poisons:  Their  Effects  and  Detection,  259,  4th  Edition,  London,  1906. 


244  ELEMENTARY   CHEMICAL   MICROSCOPY 

and  after  cooling,  gently  heat  the  material  in  a  hot  stage  of  the 
type  shown  in  Fig.  121,  until  sublimation  takes  place.  The  chief 
difficulty  with  the  tube  method  lies  in  the  fact  that  the  poor 
quality  of  the  glass,  the  striations,  air  bubbles,  and  defects 
render  the  examination  of  the  sublimate  complicated  and  diffi- 
cult. Laying  the  tube  in  a  drop  of  oil  or  of  glycerine  at  the  point 
where  the  sublimate  appears  facilitates  the  study,  by  preventing 
the  formation  of  heavy  black  contour  bands. 

Distillation.  -  -  Simple  as  well  as  fractional  distillations  are 
as  important  in  the  separation  and  identification  of  compounds 
in  microchemical  analysis  as  in  the  usual  methods  on  a  larger 
scale,  and  although  one  of  the  most  difficult  of  microchemical 
methods  may,  nevertheless,  with  care  and  patience,  be  performed 
as  successfully  as  the  series  of  fractional  distillations  on  the 
usual  scale  of  the  chemical  laboratory. 

The  simplest  of  the  distillation  problems  arises  in  the  detec- 
tion of  a  volatile  constituent  which  can  be  expelled  from  non- 
volatile material  by  heating  after  the  addition  of  a  suitable 
reagent,  as,  for  example,  in  the  detection  of  ammonia  by  expulsion 
from  material  made  alkaline  with  sodium  hydroxide  or  in  the 
detection  of  inorganic  or  organic  acids  set  free  from  their  salts  by 
phosphoric  acid  and  expelled  by  heat.  The  method  of  procedure 
is  as  follows:  Place  in  a  deep  25-millimeter  watch  glass  a  tiny 
bunch  of  fibrous  asbestos  which  has  just  been  ignited  to  redness 
by  being  held  with  the  forceps  in  the  flame  of  a  Bunsen  burner. 
In  the  absence  of  asbestos  pure  glass  wool  or  in  certain  cases  even 
a  piece  of  filter  paper  may  be  employed  as  the  absorbent,  but  if 
filter  paper  is  employed  a  blank  must  always  be  made  to  prove 
that  no  misleading  substances  result.  The  asbestos  or  glass 
wool  prevents  the  spurting  and  splashing  of  the  liquid.  Upon 
the  absorbent  is  placed  a  small  amount  of  the  material  to  be 
tested,  sufficient  water  and  enough  expelling  reagent  to  just 
thoroughly  moisten  the  mass  but  no  more.  Invert  over  the 
watch  glass  thus  prepared  a  glass  slide,  bearing  at  its  center  a 
minute  drop  of  water  about  1  millimeter  in  diameter  which  has 
been  acidulated  or  made  alkaline  as  the  case  requires.  Hold  the 
watch  glass  thus  covered   by  grasping  its  edges  between   the 


METHODS  FOR  HANDLING  SMALL  AMOUNTS  OF  MATERIAL 


245 


thumb  and  forefinger,  place  a  cooling  drop  of  water  upon  the  top 
of  the  slide  and  heat  the  watch  glass  gently  over  a  micro-flame 

(Fig.  129)  until  vapors  begin  to  con-     

dense  upon  the  object  slide.  Heating 
to  violent  boiling  must  be  avoided. 
The  cooling  drop  upon  the  upper  sur- 
face of  the  object  slide  is  removed, 
the  slide  raised  from  the  watch  glass 
and  turned  over  with  a  quick  move- 
ment. The  proper  reagents  for  dis- 
closing the  presence  of  the  constituent 
being  sought  are  added  and  the  resulting  preparation  examined 
with  the  microscope. 

The  method  just  described  is  applicable  only  to  easily  vola- 
tilized substances  and  where  prolonged  heating  is  unnecessary, 
but  even  in  expelling  ammonia,  the  fingers  become  uncomfort- 
ably hot.  To  avoid  this  discomfort  the  distilling  device  shown  in 
Figs.  130  and  131  may  be  employed.     It  consists  of  a  tiny  glass 


Fig.   129.     Watch-glass  Method 
of  Distillation. 


Fig.  130.     Apparatus  for  Microchemical  Distillations.     (Slightly  Enlarged.) 

crucible  C,  whose  upper  edge  is  ground  smooth  and  true,  a  sup- 
porting clamp  made  of  spring  brass  wire  W  and  an  ordinary 
short  object  slide  O.  The  component  parts  are  shown  in  Fig. 
131,  and  the  apparatus  in  use  in  Fig.  130.  Just  as  in  the  watch 
glass  method  fibrous  asbestos  or  glass  wool  is  employed  as  an 
absorbent,  an  acidulated  or  alkaline  drop  serves  to  retain  the 
volatile  constituent  and  a  cooling  drop  is  placed  upon  the  upper 
surface  of  the  condensing  slide.     A  lever  L  serves  to  keep  the 


246 


ELEMENTARY   CHEMICAL   MICROSCOPY 


clamp  open  when  removing  or  changing  the  object  slide  serving 
as  a  cover. 

Instead  of  holding  the  watch  glass  and  cover,  at  the  edges, 
between  the  thumb  and  finger  as  described  above,  the  clamp 
shown  in  Fig.  131  may  be  used,  or  two  watch  glasses  with  ground 


Fig.  131. 


edges  selected  to  fit  edge  to  edge  may  be  clamped  together.  In 
certain  instances  either  one  of  these  watch  glass  methods  may 
prove  to  be  more  practicable  than  the  crucible.  In  all  cases, 
however,  the  clamp  support  is  far  superior  to  the  fingers. 

Although  the  device  just  described  may  be  satisfactorily 
applied  to  the  fractional  distillation  of  small  amounts  of  volatile 
liquids,  small  distilling  tubes  of  the  form  suggested  by  Behrens  1 
will  be  found  in  certain  cases  to  be  somewhat  safer  for  very 
volatile  substances.  These  are  readily  made  from  small  glass 
tubing  of  thin  wall  as  shown  in  Fig.  132;  the  different  steps  in 
the  preparation  are  indicated  in  1,  2,  3,  4  and  5.  The  finished 
distilling  tube  is  shown  in  A.  To  introduce  the  liquid  to  be  dis- 
tilled into  one  of  the  tiny  bulbs,  fuse  the  end  of  one  of  the  project- 
ing tubes,  cool  thoroughly  and  introduce  the  end  of  the  open  tube 
into  the  drop  of  liquid,  warm  the  upper  bulb  to  drive  out  air  and 
allow  to  cool  with  the  tube  still  dipping  into  the  liquid.  If  an 
insufficient  quantity  of  the  liquid  enters,  heat  again  just  enough 
to  drive  out  all  but  a  trace  of  liquid,  then  dip  the  end  below  the 
liquid  to  be  tested  and  heat  the  lower  bulb  until  the  contents  are 

1  Anleitung  z.  mikrochem.  Anal.  (2  Auf.),  p.  140. 


METHODS  FOR  HANDLING  SMALL  AMOUNTS  OF  MATERIAL        247 

vaporized;  on  cooling  the  bulb  will  fill  with  sufficient  liquid. 
Now  open  the  fused  end  of  the  tube  and  close  that  end  of  the 
tube  through  which  the  bulb  was  filled.  Heat  the  empty  bulb 
just  enough  to  drive  out  any  material  which  may  have  found  its 
way  therein.  When  absolutely  cold,  heat  the  liquid  gently, 
distilling  it  over  into  the  empty  bulb.  The  distillate  is  expelled 
by  carefully  tipping  the  apparatus  on  its  side  and  gently  warming 
the  air  entrapped  back  of  the  distillate,  or  cut  off  the  sealed  end 
and  blow  gently. 


Fig.  132.     Steps  in  the  Preparation  of  a  Behrens  Distilling  Tube.     (Full  Size.) 


A  more  universally  applicable  distilling  tube  is  shown  in  Fig. 
133.  It  consists  essentially  of  a  tiny  tubulated  retort  with  at- 
tached receiver.  The  liquid  is  introduced  through  the  side  ami 
which  is  then  closed  with  a  tiny  plug  of  cork  or  rubber  or  by 
fusing.  Upon  heating  the  liquid  the  vapors  pass  down  the  nar- 
row inclined  tube,  are  condensed  and  collect  in  the  rounded 
receptacle.  To  prevent  loss  the  narrow  tube  between  retort 
and  receiver  may  be  wound  with  wet  filter  paper.     The  distillate 


248 


ELEMENTARY   CHEMICAL   MICROSCOPY 


is  removed  from  time  to  time  by  means  of  capillary  pipettes. 
This  little  apparatus  also  makes  a  convenient  generator  for  hydro- 
gen and  arsine  in  testing  for  arsenic. 

When  temperatures  of  vaporization  are  needed  the  bulb  con- 
taining the  liquid  can  be  introduced  into  the  hot  stage  described 


Fig.  133.     Tube  for  Microchemical  Distillations.     (Full  Size.) 

on  page  224,  the  receiving  bulb  being  kept  outside  of  the  stage 
and  cooled  with  wet  filter  paper,  the  tube  connecting  the  two 
little  bulbs  having  been  bent  at  the  proper  angle. 

Ignition,  Fusion,  etc.  —  Operations  involving  heating  to  red- 
ness are  best  performed  in  small  platinum  cups  or  spoons,  Fig. 
134,  over  the  low  flame  of  a  Bunsen  burner  or  that  of  a  miniature 
blast  lamp. 


Fig.  134.     Platinum  Cups  for  Fusions.     (Full  Size.) 


Fig.  135.     Casserole  for  Microchemical  Analysis.     (Full  Size.) 


In  the  absence  of  alkalies  tiny  cups  with  handles  made  of  fused 
silica  are  convenient,  Fig.  135;  or  tiny  porcelain  casseroles  can 
be  used.  All  the  apparatus  illustrated  are  standard  commercial 
forms  and  may  be  obtained  from  dealers  in  chemical  apparatus. 
Small  crucibles  are  occasionally  useful,  especially  those  corre- 
sponding to  No.  9  and  10  Meissan  porcelain.     Since,  however, 


METHODS  FOR  HANDLING  SMALL  AMOUNTS  OF  MATERIAL         249 

crucibles  require  a  special  support  during  ignitions  casseroles 
will  be  found  more  convenient. 

Grinding,  Crushing,  Mixing.  —  For  grinding  and  crushing 
materials  for  analysis,  the  smallest  available  agate  mortars  are 
best.     One  not  larger  than  30  millimeters  in  diameter,  Fig.  136, 


Fig.  136.     Agate  Mortar  for  Microchemical  Analysis.     (Full  Size.) 

should  be  selected.  It  must  be  carefully  scrutinized  with  a  lens 
to  see  that  its  inner  surface  is  properly  polished  and  is  free  from 
fissures,  pits  and  scratches.  A  mortar  made  from  a  first  quality 
piece  of  agate,  if  properly  cared  for,  should  last  a  lifetime. 


CHAPTER   XIII. 

THE  METHODS  OF  MICROCHEMICAL  QUALITATIVE 

ANALYSIS. 

In  order  that  success  may  follow  our  efforts  in  the  application 
of  tests  resulting  in  the  production  of  characteristic  microscopic 
crystals,  it  is  essential  that  reagents  be  always  applied  in  the  best 
possible  manner  and  in  concentrations  and  under  conditions  such 
as  will  lead  to  the  separation  of  a  solid  crystalline  phase  in  a  very 
short  period  of  time.  It  is  therefore  necessary  that  we  first 
ascertain  the  best  method  of  procedure  for  each  particular  re- 
agent. Most  of  the  failures  to  obtain  satisfactory  results  when 
attempting  microchemical  reactions  are  due  to  a  lack  of  apprecia- 
tion of  the  importance  of  this  fact.  Manuals  of  microchemical 
analysis  usually  neglect  to  state  definitely  the  best  manner  of 
adding  a  reagent  to  a  drop  to  be  tested,  assuming  that  the  in- 
vestigator will  ascertain  for  himself  the  conditions  which  will 
yield  him  products  most  easily  identified. 

Under  similar  conditions  as  to  concentration,  acidity  and 
manner  of  reagent  application,  the  crystalline  phase  will  not  only 
almost  invariably  separate  with  the  same  habit,  but  the  crystals 
will  usually  develop  to  the  same  size  and  will  lie  upon  the  object 
slide  in  each  experiment  in  the  same  positions  with  respect  to 
faces. 

The  following  methods  for  performing  microchemical  reactions 
involve  different  manipulations  and  can  be  considered  as  typical 
procedures,  each  applicable  to  the  detection  of  a  number  of  dif- 
ferent elements  or  compounds.  The  student  should  perform 
them  until  he  is  sufficiently  proficient  to  invariably  obtain  an 
unequivocal  test  and  one  yielding  each  time  similar  crystals  of  a 
similar  size.  The  more  insoluble  the  compound,  the  more 
rapidly  the  crystals  will  separate  and  the  smaller  they  will  be. 

For  convenience  for  future  reference  these  methods  are  here 
numbered  and  described  in  detail. 

250 


THE  METHODS  OF  MICROCHEMICAL  QUALITATIVE  ANALYSIS    251 

/.  A  drop  of  a  solution  of  the  reagent  is  allowed  to  flow  into  a 
drop  of  the  solution  of  the  material  to  be  tested. 

This  method  of  applying  the  reagent  is  more  often  employed 
than  any  other,  and  is  generally  far  preferable  to  the  ad- 
dition of  a  drop  of  reagent  directly  to  the  solution  to  be 
tested. 

A  perfectly  clean  object  slide  is  required.  Upon  it  near  a  cor- 
ner place  a  small  drop  of  the  solution  of  the  material  to  be  tested. 
This  drop  should  be  spread  out  until  it  attains  a  diameter  of 
approximately  5  millimeters  and  a  depth  of  not  over  half  a  milli- 
meter. A  drop  of  the  reagent  of  the  same  diameter  but  about 
twice  the  depth  is  next  placed  adjacent  to  the  first  drop  at  a  dis- 
tance of  2  to  3  millimeters.  The  concentration  of  the  reagent 
drop  should  usually  be  slightly  greater  than  that  of  the  substance 
being  tested.  By  means  of  a  platinum  wire  or  drawn-out  glass 
rod,  a  tiny  channel  is  made  to  flow  from  the  reagent  into  the 
test  drop,  the  object  slide  being  tipped  very  slightly  to  facilitate 
the  flow,  but  under  no  condition  should  the  two  drops  merge 
completely. 

Having  a  higher  concentration  in  the  reagent  drop  usually 
leads  to  a  flow  of  this  liquid  at  a  lower  level  and  therefore  close 
to  the  object  slide  because  of  a  slightly  greater 
density  than  that  of  the  solution  of  the  sub- 
stance.    Crystals  thus  tend  to  form  upon  the 
slide  instead  of  floating  about  in  the  liquid. 
The  more  perfect  crystal  faces  are  on   the 
upper  side,  or,  in  other  words,  that  side  most 
easily  studied  by  means  of  the  microscope. 
Crystals    which    float    about    usually    grow 
downwards  from    the  upper  surface  of    the 
test   drop   and  therefore   have    the  well-de- 
veloped faces  on  their  under  side,  which  must  remain  more  or 
less  invisible. 

The  maximum  sizes  of  drops  are  shown  in  the  diagram,  I  i :_r - 
137.  The  reagent  drop  R  has  been  made  to  flow  into  the  drop 
to  be  tested  S  through  a  tiny  channel  c.  The  crystalline  phase 
constituting  the  identity  test  separates  at  p. 


252  ELEMENTARY   CHEMICAL   MICROSCOPY 

EXPERIMENTS. 

a.  Addition  of  Chloroplatinic  Acid  (platinum  chloride)  to  a  solution  of  a 
potassium  salt  (KG).  Application :  testing  for  K,  NH4,  Rb,  Cs,  Na,  many  organic 
bases,  etc. 

Repeat  the  experiment,  using  a  fragment  of  CsCl  in  a  drop  of  the  same  size 
as  that  of  the  potassium  salt  just  employed.  Note  the  instantaneous  forma- 
tion of  a  precipitate  and  that  crystals  are  very  much  smaller.  Repeat  again,  using 
a  very  dilute  solution  of  CsCl.  Next  try  a  solution  drop  of  KC1  containing  very 
little  CsCl.  Allow  to  evaporate  spontaneously  after  the  addition  of  the  reagent, 
Cs  separates  first,  then  K. 

b.  Addition  of  Ammonium  Mercuric  Sulphocyanate  to  a  dilute  solution  of 
a  copper  salt. 

c.  Addition  of  a  solution  of  a  Tartrate  to  a  solution  of  CaCl2  acidulated  with 
HC2H3O2. 


77.    The  substance  to  be  tested  is  added  to  a  drop  of  the  reagent. 

This  method  of  applying  tests  is  the  one  least  often  employed. 
It  will  prove  successful  in  such  reactions  as  require  for  the  sepa- 
ration and  characteristic  development  of  the  crystalline  phase  a 
constant  addition  of  one  component,  in  this  case  that  to  be 
tested  for  in  small  but  almost  uniform  amount. 

The  fragment  of  material  is  added  to  the  center  of  a  shallow 
broad  drop.  Warming  gently  will  accelerate  the  separation  of 
crystals. 

EXPERIMENTS. 

a.  To  a  drop  of  a  solution  of-Bi2(S04)3  containing  a  trace  of  free  HN03,  add 
a  fragment  of  K2SO4. 

Applications  —  Testing  for  K,  for  Na,  for  Bi,  etc. 


III.  A  tiny  fragment  of  the  solid  reagent  is  added  to  a  drop  of  the 
solution  of  the  substance  to  be  tested. 

This  case  is  substantially  similar  to  Method  II,  and  is  governed 
by  the  same  general  conditions.  It  will  be  found  to  be  the  safest 
procedure  in  nearly  all  reactions  where  the  solid  phase  at  first 
formed  is  soluble  in  excess  of  the  reagent,  for  there  will  always  be 
during  an  appreciable  time  (owing  to  the  rather  slow  solution  of 
the  reagent)  a  zone  in  which  the  equilibrium  is  such  that  the  solid 
phase  can  exist.  Thus  the  fragment  of  reagent  will  be  surrounded 
by  a  clear  space  or  ring,  at  the  outer  edge  of  which  the  solid 


THE  METHODS  OF  MICROCHEMICAL  QUALITATIVE  ANALYSIS    253 

crystalline  phase  will  easily  be  distinguished  under  the  micro- 
scope. If  the  fragment  of  reagent  added  is  too  large,  the  clear 
ring  rapidly  increases  in  diameter  as  the  reagent  dissolves,  and 
the  solid  phase  is  correspondingly  rapidly  forced  toward  the  cir- 
cumference of  the  test  drop  and  eventually  disappears  completely. 
The  test  drop  should  be  somewhat  deeper  than  usual  and  should 
cover  a  relatively  small  area. 

Reactions  involving  no  re-solution  of  the  crystals  first  sepa- 
rating require  no  such  careful  attention  to  equilibrium  conditions, 
nor  do  they  necessitate  such  constant  observation  under  the 
microscope  in  order  that  the  progress  of  the  reaction  may  be 
followed.  In  this  class  fall  the  precipitations  of  one  metal  by 
another  metal  which  is  more  electropositive.  If,  for  example,  we 
make  use  of  the  electrochemical  series  of  Wilsmore-Ostwald,1  it 
is  found  that  the  metallic  elements  are  arranged  thus: 

+  «-  Mg,  Al,  Mn,  Zn,  Cd,  Fe,  Tl,  Co,  Ni,  Sn,  Pb, 
(H),  Cu,  As,  Bi,  Sb,  Hg,  Ag,  Pd,  Pt,  Au,-»  -. 

Theoretically  each  element  in  this  series  is  able  to  replace  the 
elements  below  it  in  the  series  which  are  less  electropositive. 
Since  in  many  instances  the  metal  displaced  will  separate  in 
characteristic  crystalline  form,  the  addition  of  a  tiny  piece  of  Mg 
or  of  Al  to  a  very  slightly  acidified  drop  may  be  made  to  yield  a 
beautiful  test  for  metals  farther  along  in  the  series.  This  type 
of  reaction  is  also  of  great  value  in  effecting  separations  prior  to 
the  application  of  identity  tests,  or  in  the  separation  of  elements 
which  may  interfere  with  future  testing. 

A  knowledge  of  the  electrochemical  series  is  absolutely  essential 
in  all  analyses  of  alloys  where  tiny  fragments  are  not  completely 
dissolved  since  there  will  be  solution  of  one  or  more  components 
and  the  precipitation  of  others  upon  the  surface  of  the  undissolved 
material.  Furthermore,  a  study  of  the  above  series  will  reveal 
at  once  the  fact  that  the  addition  to  a  test  drop  of  a  reagent  with 
reducing  properties  will  in  all  likelihood  be  followed  by  the  par- 
tial precipitation  of  any  metals  present  which  fall  in  the  electro- 
negative end  of  the  series. 

1  Zeit.  phys.  Chem.,  36  (1901)  92. 


254  ELEMENTARY   CHEMICAL  MICROSCOPY 

777  A .  A  tiny  drop  of  the  reagent  is  added  directly  to  the  test  drop 
at  its  center. 

This  procedure  is  effective  in  all  cases  where  the  crystalline 
phase,  which  is  wished,  is  not  too  slowly  formed,  has  great  crystal- 
lizing powers  and  forms  a  large  molecule.  It  may  be  said,  that, 
in  a  general  way,  the  addition  of  a  drop  of  the  reagent  directly 
to  the  drop  to  be  tested  is  applicable  to  practically  all  micro- 
chemical  reactions.  But  in  many  special  cases  the  crystals 
separating  are  not  as  characteristic  nor  as  constant  in  their  habit 
as  in  other  methods,  nor  does  the  reaction  take  place  with  suffi- 
cient rapidity. 

The  direct  addition  of  the  reagent  is  also  practiced  when 
a  heavy  agglutinated  precipitate  results,  which  must  subse- 
quently be  freed  from  its  supernatant  liquid  and  then  recrys- 
tallized. 

The  most  frequent  cases  where  reagent  drops  are  added  are 
in  acidification,  alkalinization,  neutralization;  and  in  the  addi- 
tion of  some  reagent  whose  purpose  is  to  mitigate  the  dele- 
terious action  of  some  compound  present,  as,  for  example,  the 
addition  of  sodium  or  ammonium  acetate  to  prevent  a  free  mineral 
acid  from  interfering  with  a  test.  Usually,  however,  a  fragment 
of  the  solid  acetate  is  added  rather  than  a  drop  of  solution.  Or 
we  may  add  a  drop  of  glycerine  solution  to  retard  the  formation 
of  certain  crystals. 

EXPERIMENTS. 

a.  To  a  drop  of  a  dilute  solution  of  HgCl2  add  a  fragment  of  KI.  Note  the 
kind  of  crystals  formed  and  their  position  with  respect  to  the  fragment  of  KI. 
After  the  fragment  of  KI  has  dissolved  leaving  a  clear  area,  add  to  its  center  a 
tiny  fragment  of  CuS04;  the  Hgl2  which  has  dissolved  will  be  reprecipitated. 

b.  To  a  drop  of  a  very  dilute  solution  of  HAuCL,  (chloroauric  acid)  add  a 
tiny  fragment  of  T1N03.  In  this  case  the  characteristic  crystals  consisting  of 
TIAuCL,  •  5  H20  (?)  form  upon  the  fragments  of  the  reagent. 

c.  To  a  drop  of  PbN03  solution  add  a  tiny  drop  of  a  dilute  solution  of  CuS04. 
Stir.  Add  a  fragment  of  Na(C2H302) ,  stir  until  almost  dissolved.  Now  add  a 
fragment  of  KN02  and  follow  with  a  trace  of  dilute  HC2H302.  Tiny  black  cubes 
of  the  triple  salt  2  (KN02)  •  Cu(N02)2  •  Pb(N02)2  separate. 

d.  To  a  drop  of  a  solution  of  PbN03  add  a  tiny  fragment  of  metallic  mag- 
nesium.    Try  in  like  manner  a  number  of  elements  in  the  electrochemical  series. 


THE  METHODS  OF  MICROCHEMICAL  QUALITATIVE  ANALYSIS    255 

IV.  The  reagent  solution  is  drawn  in  a  narrow  channel  across  a 
dry  film  obtained  by  evaporating  to  dryness  a  solution  of  the  sub- 
stance to  be  tested. 

Reactions  requiring  a  nice  adjustment  of  concentration  or 
leading  to  the  formation  of  moderately  soluble  compounds,  thus 
entailing  a  considerable  loss  of  time  waiting  for  the  formation  of 
crystals,  if  much  liquid  were  present,  are  always  best  performed 
on  the  dry  residue.  Residues  for  such  reactions  should  consist 
of  thin,  uniform  films  of  material  and  are  to  be  obtained  only 
when  scrupulously  clean  slides  are  employed,  when  only  a  small 
amount  of  the  substance  is  present  and  when  care  is  taken  to 
avoid  heating  too  hot  during  the  evaporation.  Gentle  heating  and 
blowing  on  the  warm  drop  will  give  the  best  results.  Heating 
should  be  done  at  the  corner  of  the  object  slide  over  the  tiny  flame 
of  the  micro-burner,  tipping  the  object  slide  so  as  to  cause  the 
drop  to  flow  toward  the  corner  and  holding  above  the  flame  in  such 
a  position  that  the  tip  of  the  flame  is  nearer  the  middle  of  the 
slide.   This  prevents  the  liquid  from  creeping  and  from  spreading. 

It  is  usually  advisable  to  examine  this  film  under  a  low  power 
to  learn  whether  it  is  thin  and  uniform  in  character. 

In  cases  where  a  ridge  of  the  solid  material  tends  to  form  around 
the  edge,  as  will  be  the  case  if  too  much  substance  has  been  used, 
it  is  advisable  to  remove  this  ridge  by  means  of  the  platinum 
spatula  (Fig.  68),  using  it  shovel-wise.  The  reagent  is  dissolved 
in  a  tiny  drop  of  water  placed  just  beside  the  dried  test  drop,  and 
is  then  drawn  across  the  latter  with  a  quick  stroke  of  a  glass 
rod  with  drawn-out  end,  care  being  taken  to  avoid  rubbing  the 
slide  in  leading  the  reagent  across.  To  facilitate  the  flow,  the 
slide  should  be  inclined  a  trifle  in  the  direction  the  liquid  is  being 
drawn.  The  solution  should  never  spread  over  the  entire  film 
of  substance,  but  should  remain  as  a  streak  of  liquid  dividing  the 
dry  spot  in  half.  When  the  liquid  completely  covers  the  residue, 
it  is  usually  due  to  one  or  more  of  several  causes:  too  thick  a  film ; 
a  slide  that  is  not  clean;  heating  after  the  residue  was  dry  and 
so  detaching  it  from  the  glass;  too  much  reagent,  or  the  presence 
of  excessively  soluble  compounds  or  those  which  refuse  to  adhere 
to  the  glass. 


256  ELEMENTARY   CHEMICAL  MICROSCOPY 

EXPERIMENTS. 

a.  Obtain  a  thin  uniform  film  of  NaCl  as  described  above. 

b.  Near  the  residue  (2  to  3  millimeters)  place  a  drop  of  distilled  water;  acidify 
the  drop  by  touching  with  a  drawn-out  glass  rod  which  has  been  dipped  in  dilute 
HC2H3O2;  introduce  a  tiny  fragment  of  U^CCitLX^^.  Warm  the  drop  gently  to 
facilitate  solution,  but  do  not  evaporate.  Cool.  By  a  single,  rapid  stroke  of  a 
glass  rod  or  platinum  wire,  draw  a  streak  or  channel  of  the  reagent  across  the 
center  of  the  dry  material.  Place  the  preparation  upon  the  stage  of  the  micro- 
scope and  search  the  edges  of  the  streak  of  liquid  at  once.  Tiny  faintly  yellow 
triangular  and  tetrahedral  crystals  of  NaC2H302  •  U02(C2H302)2  will  be  seen. 

Analytical  applications  —  Na,  Mg,  U,  acetates. 


V.  Upon  failure  to  obtain  a  decisive  test  owing  to  the  unsatis- 
factory separation  of  crystals,  the  delicacy  of  the  reaction  can  be 
increased  through  the  addition  of  another  reagent  which  will  produce 
a  less  soluble  salt  of  the  same  nature. 

The  chemical  reactions  involved  in  the  practical  application 
of  this  method  of  increasing  the  delicacy  of  microchemical  iden- 
tity-tests are  among  the  most  interesting  and  instructive  with 
which  we  have  to  deal.  To  properly  apply  and  interpret  them 
or  to  devise  new  tests  to  meet  special  conditions  requires,  in 
inorganic  chemistry,  a  good  working  knowledge  of  the  Periodic 
System  of  Mendelejeff:  while  in  the  case  of  reactions  in  the  field 
of  organic  chemistry  success  can  only  follow  a  profound  knowl- 
edge of  the  chemical  and  physical  properties  of  the  compounds 
to  be  studied. 

Considering  the  method  only  from  the  viewpoint  of  inorganic 
analysis,  the  delicacy  of  a  test  can  be  increased  by  introducing 
into  the  test  drop,  in  which  no  separation  of  a  crystalline  phase 
has  taken  place,  a  salt  whose  base  will  form  a  less  soluble  com- 
pound than  that  originally  present.  For  example,  suppose  a 
test  for  the  presence  of  chlorides  is  being  made  by  means  of 
platinum  sulphate  and  a  salt  of  potassium;  with  much  chlorine, 
potassium  chloroplatinate  will  separate,  but  if  we  obtain  no 
crystals,  we  may  add  a  little  rubidium  sulphate  to  the  drop. 
Should  this  yield  no  result,  it  can  be  followed  by  a  little  cesium 
sulphate  and  finally  carried  to  the  limit  by  the  introduction  of  a 
thallous  salt.  With  the  potassium  salt  the  limit  of  the  test  is  y~4 
milligrams  of  chlorine,  but  with  thallium  4-6  milligrams  (Beh- 


THE  METHODS  OF  MICROCHEMICAL  QUALITATIVE  ANALYSIS    257 

rens).  That  is  to  say,  that  while  we  may  obtain  proof  of  the 
presence  of  an  exceedingly  minute  amount  of  chlorine  through 
the  separation  of  crystals  of  thallous  chloroplatinate,  approxi- 
mately one  hundred  times  as  much  chlorine  must  be  present  in 
order  that  it  may  be  revealed  as  potassium  chloroplatinate. 

This  plan  of  producing  a  less  soluble  salt  is,  in  general,  to  be 
preferred  to  that  of  causing  the  separation  of  a  solid  phase  by 
forcing  back  the  dissociation,  by  means  of  strong  acids,  salting- 
out,  or  other  similar  processes,  since  well-formed  crystals  result 
in  the  first  case,  but  abnormal,  atypical  salts  are  apt  to  appear 
in  the  other  cases. 

EXPERIMENTS. 

Repeat  Experiment  IIIc,  page  254,  gradually  reducing  the  concentrations  until 
no  triple  salt  separates,  then  add  a  fragment  of  CsCl;  the  triple  nitrite  of  Cs,  Cu 
and  Pb  will  appear.  In  a  new  preparation  carry  the  dilution  a  little  farther,  so 
that  the  Cs  salt  does  not  appear  at  once.  Add  a  fragment  of  T1X03.  The  deli- 
cacy of  the  reaction  will  be  approximately  K:T1  as  3:  1. 


VI.  The  reaction  can  be  hastened  and  the  delicacy  of  the  test 
increased  by  exposure  to  alcohol  vapors. 

It  was  stated  under  Method  V,  that  it  is  rarely  desirable 
to  employ  a  reagent  that  will  force  back  the  dissociation;  the 
reasons  being  that  the  addition  of  such  a  reagent  causes  a  too 
rapid  separation  of  a  solid  phase  and  there  is  a  tendency  towards 
the  production  of  malformed,  skeleton  or  exceedingly  tiny  crys- 
tals. When,  however,  the  separation  of  a  solid  phase  is  acceler- 
ated by  the  gradual  absorption  of  a  vapor  in  the  test  drop,  thus 
reducing  the  solubility  by  forcing  back  the  dissociation  very 
slowly,  it  requires  only  a  little  care  to  assure  the  separation  of 
characteristic,  well-formed  crystals. 

Alcohol  is  exceptionally  well  fitted  for  use  in  all  cases  where  a 
crystalline  compound  is  less  soluble  in  alcohol  than  in  water. 

One  of  two  methods  will  be  found  convenient.  Place  near  the 
test  drop  a  small  piece  of  filter  paper.  Saturate  the  paper  with 
a  drop  or  two  of  alcohol,  carefully  avoiding  the  addition  of  more 
than  the  paper  will  absorb.  Cover  the  drop  and  paper  with  a 
watch  glass  (Behrens) ;    or  place  a  piece  of  paper  at  the  bottom 


258  ELEMENTARY   CHEMICAL   MICROSCOPY 

of  a  crucible,  preferably  a  tiny  glass  crucible  as  described  on 
page  245,  or  in  a  small  beaker.  Saturate  with  alcohol  and  invert 
over  the  test  drop.  Owing  to  the  difference  in  the  vapor  tensions, 
alcohol  will  be  absorbed  by  the  aqueous  solution  and  the  crystal- 
line phase  will  rapidly  separate.  Only  a  very  short  exposure  is 
necessary. 

When  dealing  with  very  thin  films  or  tiny  drops  where  there 
is  a  tendency  to  evaporate  to  dryness,  exposure  to  alcohol 
vapors  is  especially  valuable. 

EXPERIMENTS. 

a.  Prepare  a  large  drop  of  a  moderately  concentrated  solution  of  PbN03. 
From  this  large  drop  take  two  small  ones.  Allow  one  of  them  to  evaporate  spon- 
taneously. Treat  the  other  with  alcohol  vapor  as  described  above.  Note  the 
difference  in  time  required  for  the  appearance  of  crystals. 

b.  To  a  dilute  solution  of  a  calcium  salt  add  a  drop  of  dilute  H2S04  by  Method  I, 
page  251.  Sheaves,  bundles  and  isolated  acicular  crystals  of  CaS04  •  2  H20  will 
separate.  Prepare  a  solution  of  the  calcium  salt  so  dilute  that  no  CaS04  appears 
after  standing  two  or  three  minutes.  Expose  to  alcohol  vapors  and  note  that 
characteristic  crystals  are  soon  visible. 


VII.  The  reagent  is  dissolved  in  alcohol  and  a  drop  of  the  alco- 
holic solution  is  employed  as  in  Method  I. 

Although  we  are  here  dealing  with  a  mode  of  applying  the 
reagent  already  discussed,  alcoholic  solutions  need  special  men- 
tion because  of  the  care  required  in  their  application.  The  re- 
marks which  follow  are  equally  applicable  to  any  other  solvents 
or  reagents  of  lower  boiling  point  than  water  or  of  different  sur- 
face tensions. 

There  is  always  a  marked  tendency  of  the  alcoholic  reagent  to 
spread  over  the  whole  object  slide,  carrying  with  it  the  drop  of 
solution  to  be  tested,  or  breaking  the  latter  up  into  so  many  drop- 
lets as  to  render  reliable  observations  impossible.  Not  infre- 
quently considerable  skill  is  essential  to  prevent  this  dissipation 
of  material. 

When  an  alcoholic  reagent  must  be  added  to  a  reagent  drop, 
always  have  the  drop  at  the  corner  of  the  slide,  and  tip  the  slide 
slightly  before  the  alcohol  solution  is  applied  to  the  glass  near 
the  drop;  as  the  reagent  leaves  the  rod  or  pipette  increase  the 


THE  METHODS  OF  MICROCHEMICAL  QUALITATIVE  ANALYSIS    259 

inclination  of  the  slide  at  once  so  as  to  cause  the  reagent  to  flow 
toward  the  material  to  be  tested.  Counteract  any  tendency  of 
the  reagent  to  creep  up  by  immediately  increasing  the  inclina- 
tion to  an  almost  vertical  position. 

Often  the  preparation  cannot  be  laid  flat  upon  the  stage  be- 
cause of  the  instant  spreading  of  the  alcoholic  solution.  In  such 
an  event,  the  corner  of  the  object  slide  holding  the  liquid  is  in- 
serted in  the  stage  opening  and  may  be  held  in  place  by  another 
slide  placed  upon  the  stage,  carrying  a  piece  of  "plasticine" 
against  which  the  inclined  slide  is  pressed.  The  preparation  can 
then  be  examined  with  a  low  power,  focusing  each  different  area 
as  it  is  brought  into  the  field  by  means  of  the  stage  centering 
screws. 

Because  of  the  difficulties  involved  in  the  study  of  inclined 
preparations  it  is  always  better  to  first  evaporate  to  dryness  the 
drop  of  material  to  be  tested  so  as  to  obtain  a  broad  thin  film 
(see  Method  IV)  and  use  a  reagent  solution  made  with  as  dilute 
alcohol  as  will  yield  the  proper  conditions  required  in  the  test. 

EXPERIMENTS. 

a.  Obtain  a  thin  film  of  KG  at  the  corner  of  an  object  slide.  Place  near  by 
a  drop  of  an  alcoholic  solution  of  freshly  prepared  sodium  bismuth  thiosulphate.1 
Tip  the  slide  slightly  and  draw  the  reagent  across  the  dry  film. 

Yellow  monoclinic 2  crystals  of  potassium  bismuth  thiosulphate  separate.  The 
salt  is  believed  to  have  the  formula 

3(K2S203)-Bi2(S203)3-2H20. 
It  is  readily  soluble  in  water,  almost  insoluble  in  alcohol. 

1  The  reagent  is  prepared  as  follows:  Place  in  a  small  watch  glass  (25  mm.)  a 
small  drop  of  dilute  hydrochloric  acid;  add  repeatedly  minute  amounts  of  basic 
bismuth  nitrate,  warming  gently  from  time  to  time  and  stirring  thoroughly,  until 
a  trace  of  the  basic  nitrate  remains  undissolved;  now  add  a  bare  trace  of  hydro- 
chloric acid;  just  sufficient  to  dissolve  the  little  residue  of  bismuth  salt,  but  no  more; 
then  add  to  the  preparation  a  tiny  drop  of  water.  A  permanent  precipitate  of 
bismuthyl  chloride  should  result.  If  the  first  drop  of  water  does  not  produce 
a  permanent  precipitate,  another  drop  must  be  added.  To  this  latter  turbid  solu- 
tion a  saturated  solution  of  sodium  thiosulphate  is  carefully  added,  with  constant 
stirring,  a  tiny  drop  at  a  time,  as  long  as  any  of  the  precipitate  remains  undis- 
solved. An  excess  of  sodium  thiosulphate  is  to  be  avoided.  A  perfectly  clear, 
faintly  yellowish  solution  should  result.  To  this  clear  liquid  add  alcohol  (95  per 
cent)  drop-wise,  until  a  permanent  turbidity  results,  which  is  in  turn  cleared  up 
by  the  addition  of  a  single  drop  of  water. 

2  Hiiysse,  Zeit.  anal.  Chem.,  39  (1900),  9. 


260  ELEMENTARY   CHEMICAL   MICROSCOPY 

b.  Prepare  a  film  of  KG.  Draw  across  it  an  alcoholic  solution  of  picric  acid 
Cel^NO^aOH.  Potassium  picrate  C6H2(N02)3OK  is  obtained  in  long  acicular 
prisms  of  the  orthorhombic  system.  Try  in  like  manner,  Na,  NH4  and  Cs  chlorides. 
Try  with  Na2C03. 


VIII .  The  reagent  is  incorporated  into  a  fiber  of  silk,  cotton, 
wool,  or  in  a  filament  of  guncotton  and  the  prepared  fiber  dipped 
into  the  drop  of  solution  to  be  tested. 

The  development  of  the  methods  for  testing  by  means  of 
textile  fibers  into  which  are  incorporated  the  reagents  to  be 
employed,  is  due  to  Emich  1  and  to  Donau.2 

That  variety  of  fiber  is  chosen  which  has  the  highest  adsorptive 
power  for  the  specific  reagent  to  be  used,  as,  for  example,  silk  for 
adsorbing  litmus;  wool  or  silk  for  turmeric;  silk  or  cotton  for 
gold;  guncotton  for  adsorbing  zinc  sulphide,  etc. 

Two  methods  of  applying  the  reagent  fiber  to  the  test  drop  are 
in  vogue;  one  consists  in  laying  the  fiber  across  the  drop  of  solu- 
tion so  that  about  two-thirds  of  its  length  will  be  outside  the 
drop.  The  liquid  is  drawn  by  the  capillarity  of  the  fiber  so  that 
it  gradually  flows  over  its  whole  length.  The  second  method 
consists  in  rolling  a  bit  of  beeswax  between  the  fingers  until  a 
tiny  slender  cone  is  obtained  about  10  millimeters  long  by  2  or  3 
millimeters  in  diameter.  One  end  of  the  reagent  fiber  is  attached 
to  the  apex  of  the  wax  cone  and  the  base  of  the  cone  is  gently 
pressed  against  an  object  slide.  A  very  minute  rounded  drop 
of  the  solution  to  be  tested  is  placed  upon  the  slide  about  5  milli- 
meters away  from  the  base  of  the  cone;  the  cone  is  then  bent 
over  until  the  free  end  of  the  fiber  dips  into  the  liquid.  The 
preparation  is  next  placed  upon  the  stage  of  the  microscope  and 
the  instrument  focused  upon  the  fiber  just  above  the  drop. 
Through  capillarity  the  liquid  is  drawn  upon  the  fiber  and  the 
reaction  resulting  is  easily  recognized. 

1  Emich,  Monats.,  22  (1001),  670;  23  (1902),  76;  Ann.,  351  (1907),  426. 
*  Donau,  Monats.,  25  (1904),  545;  Ann.,  351  (1907),  432. 


THE  METHODS  OF  MICROCHEMICAL  QUALITATIVE  ANALYSIS    261 

Applications  of  this  Method. 

Testing  for  acidity  or  alkalinity Litmus-silk 

Differentiating  between  strong  mineral 

acids  and  organic  acids Congo-red-silk 

Testing  for  boric  acid,  borates Turmeric-wool 

Group  reagent  for  the  heavy  metals. .  .  .  Guncotton-zinc-sulphide 
Test  for  gold .  .  .  .Adsorption  upon  silk,  reduction  with  stannous 

chloride 
For  the  methods  for  preparing  the  fibers,  see  page  271. 

EXPERIMENTS. 

a.  Test  a  very  dilute  drop  of  an  acidulated  solution  with  blue  litmus-silk. 

b.  Test  a  dilute  drop  of  alkaline  solution  with  red  litmus-silk. 

c.  Place  a  drop  of  a  dilute  solution  of  borax  upon  an  object  slide,  acidulate 
with  dilute  HC1.  Dip  into  the  drop  from  a  wax  cone  a  fiber  of  turmeric-wool. 
Allow  to  evaporate  spontaneously  to  dryness.  Examine  the  fiber  under  the 
microscope.  It  should  have  a  brownish  color.  Lay  the  fiber  upon  a  slide  and 
moisten  with  a  10  to  15  per  cent  solution  of  NaOH.  If  borates  were  present  the 
fiber  turns  a  bluish  or  lavender  color. 

d.  Into  a  tiny  drop  of  a  solution  containing  Au,  lay  a  fiber  of  purified  raw  silk, 
warm  gently  until  evaporated  to  dryness;  carefully  avoid  too  high  a  temperature. 
The  fiber  turns  yellow  or  red.  Treat  with  a  dilute  solution  of  SnOa  containing  a 
little  tannic  acid.  A  purple  color  results,  due  to  the  precipitation  of  metallic  gold. 
The  beautiful  red  color  of  the  silk  fiber  before  the  reducing  agent  is  added  is  due  to 
colloidal  gold;  the  agglutination  of  the  colloidal  particles  by  the  SnCU  gives  rise 
to  larger  particles  which  appear  purple. 


IX.  The  delicacy  of  the  test  is  increased  by  taking  advantage  of 
adsorption  phenomena,  or  the  test  itself  depends  upon  the  adsorptive 
properties  of  a  compound. 

Although  reactions  of  this  type  are  those  most  frequently 
employed  in  the  differentiation  of  structures,  tissues,  cells  and 
cell  contents  in  biology,  histology  and  pathology,  through  the 
use  of  differentiating  stains  or  dyes,  their  applications  in  the 
chemical  laboratory  to  the  common  problems  of  qualitative 
analysis  are  limited. 

The  basis  for  selecting  a  reaction  involving  adsorption  phe- 
nomena or  solid  solution  is  that  the  resulting  reaction  shall  con- 
fer upon  a  practically  colorless  body  a  color  of  sufficient  intensity 
to  render  it  more  easily  discerned.     Whenever,  therefore,  stain- 


262  ELEMENTARY   CHEMICAL   MICROSCOPY 

ing  or  coloring  can  be  quickly  and  simply  accomplished,  advan- 
tage should  at  once  be  taken  of  the  fact. 

As  examples  of  qualitative  tests  which  may  be  considered  as 
falling  under  this  method,  the  following  may  be  cited : 

In  testing  for  perchlorates,  the  addition  of  a  permanganate 
will  yield  colored  perchlorate  crystals. 

Iodine  and  bromine  are  revealed  by  their  coloring  starch 
granules,  or  the  presence  of  a  compound  setting  free  iodine  from 
an  iodide  or  from  an  iodate  is  ascertained  by  starch.  Or,  on  the 
other  hand,  starch  is  easily  differentiated  from  other  substances 
by  staining  with  an  iodine  solution. 

Most  oil  or  fat  globules  may  be  stained  by  alkanin. 

Fullers  earth  affords  a  simple  means  of  distinguishing  between 
vegetable  and  aniline  dyes  and  in  a  few  cases  between  certain 
aniline  dyes  themselves. 

In  the  microchemical  examinations  of  rock  sections,  aluminum 
hydroxide  can  be  stained  with  congo  red  and  gelatinous  silica 
with  malachite  green  —  tests  which  may  be  employed  in  testing 
for  "weathering,"  etc. 

EXPERIMENTS. 

a.  Next  to  a  drop  of  a  dilute  solution  of  HCIO*  ot  NH4CIO4,  place  a  drop 
of  RbCl  solution  (or  KC1,  if  no  Rb  is  obtainable).  Cause  the  Rb  to  flow  into 
the  perchlorate  (Method  /).  In  a  few  seconds  colorless,  characteristic  crystals  of 
RbC104  separate.  Place  a  drop  of  dilute  KMnO*  next  to  the  preparation  and 
cause  it  to  flow  into  it.  The  crystals  of  RbC104  will  become  colored  pink.  The 
resulting  compound  is  a  solid  solution  (isomorphous  mixture)  of  the  permanganate 
in  the  perchlorate,  due  to  adsorption. 

b.  To  a  drop  of  a  dilute  KI  solution  add  a  few  granules  of  potato  or  arrow-root 
starch.  Stir.  Examine  under  the  microscope.  Add  at  the  center  a  very  minute 
fragment  of  pure  KN02  or  NaN02.  Examine  again.  The  starch  granules  should 
appear  at  the  most  only  very  slightly  colored.  Add  a  trace  of  very  dilute  HC2H3O2 
or  Ii2S04.  The  starch  granules  turn  blue  or  purple,  due  to  adsorption  of  liberated 
iodine. 

Repeat  the  experiment,  substituting  a  bromide  for  the  iodide  and  (NH4)2S208  for 
the  KN02  

X.  The  reagent  dissolved  in  a  volatile  solvent  is  spread  in  a  film 
upon  an  object  slide  in  such  a  manner  as  to  yield  a  coating  or  varnish 
non-crystalline  in  character,  and  across  this  prepared  surface  a 
solution  of  the  unknown  material  is  drawn. 


THE  METHODS  OF  MICROCHEMICAL  QUALITATIVE  ANALYSIS    263 

Behrens  1  has  successfully  used  this  procedure  in  testing  for 
the  alkaloid  quinine.  Although  no  other  practical  application 
of  this  method  of  testing  has  yet  been  made,  its  possibilities  in 
organic  analysis  are  great,  and  the  principle  upon  which  the  test 
is  based  is  exceedingly  interesting,  namely,  inducing  crystalliza- 
tion in  an  amorphous  mass  through  the  presence  of  a  mother 
substance  dissolved  in  a  suitable  solvent. 

EXPERIMENTS. 

a.  Dissolve  a  little  quinine  in  dilute  H2S04,  add  to  the  drop  of  solution  a 
fragment  of  KI,  stir  until  dissolved,  then  add  a  fragment  of  KN02.  Decant  from 
the  brown  amorphous  mass,  wash  the  precipitate  once  with  water  and  dissolve  it 
in  C2H5OH.  The  alcoholic  solution  is  flowed  over  a  clean  previously  warmed  slide 
so  as  to  cover  it  with  a  thin  homogeneous  varnish.  Examine  under  the  micro- 
scope and  make  certain. there  are  no  pleochroic  crystals. 

Dissolve  a  little  quinine  sulphate  in  dilute  HC2H3O2.  Draw  a  narrow  streak  of 
this  solution  across  the  varnished  surface.  Immediately  highly  pleochroic  crystals 
of  iodo-quinine  sulphate  (Herapathit)  will  separate. 

The  student  should  satisfy  himself  that  this  is  actually  an  excellent  test  for 
quinine,  although  quinine  was  employed  in  making  the  reagent.  Try,  for  example, 
pure  cinchonine,  cinchonamine,  etc.,  in  HC2H302  solution;  no  Herapathit  crystals 
will  be  obtained. 


XI.  Testing  for  the  evolution  of  gas  from  a  substance  when  treated 
with  a  reagent. 

Dissolve  in  hot  freshly  drawn  distilled  water  such  an  amount 
of  pure  gelatin  (one  or  two  square  millimeters  of  sheet  gelatin) 
that  the  solution  just  jells  on  cooling.  It  is  essential  that  this 
jelly  shall  not  possess  too  high  a  setting  power  nor  yet  be  so  thin 
that  considerable  time  is  required  for  it  to  set  after  melting. 

The  substance  to  be  tested,  if  a  solution,  should  be  evaporated 
to  dryness  in  a  thin  film,  or  if  a  solid,  very  finely  powdered  or 
spread  out  in  a  thin  uniform  layer.  Upon  the  dry  residue  a 
small  drop  of  the  melted  gelatin  is  caused  to  fall,  is  quickly 
spread  in  a  thin  layer,  and  the  slide  allowed  to  stand  upon  a  cool 
metal  surface  until  the  gelatin  sets.  The  preparation  is  then 
placed  upon  the  stage  of  the  microscope  and  is  focused.  Next  to 
the  jelly  drop  is  placed  the  reagent  whose  effect  is  to  be  tested, 

1  Anleitung,  z.  mikro.  Anal.  v.  wichtigsten  organ.  Verbind.  Heft  III,  92. 


264  ELEMENTARY   CHEMICAL   MICROSCOPY 

and  by  means  of  a  glass  rod,  the  reagent  drop  is  caused  to  touch 
the  jelly  mass.  The  reagent  slowly  penetrating  into  the  jelly 
attacks  the  substance.  If  a  gas  of  relatively  low  solubility  is 
generated  tiny  gas  bubbles  will  appear  in  the  gelatin. 

Applied  as  above  described  the  test  has  a  somewhat  wider 
range  of  usefulness  than  if  the  reagent  (acid)  is  dissolved  in  the 
gelatin,  as  suggested  by  Behrens. 

In  the  event  that  the  gas  set  free  by  the  reagent  is  very  soluble 
in  water,  no  gas  bubbles  will  appear;  in  such  an  event  the  gela- 
tin may  be  made  the  carrier  of  some  reagent  upon  which  the  gas 
will  react  and  be  thus  made  to  reveal  its  presence. 

EXPERIMENTS. 

a.  Evaporate  a  drop  of  Na2C03  solution.     Cover  with  gelatin,  test  with  HC1. 

b.  Place  a  little  CaC03  on  an  object  slide,  cover  and  test  as  above. 

c.  Test  a  little  zinc  dust  in  like  manner. 

J.    Test  a  cyanate  in  like  manner,  using  H2SO4. 


XII.  An  amorphous  precipitate  is  j'ormed  by  the  reagent  and 
requires  special  treatment  to  induce  crystallization. 

It  has  already  been  pointed  out  that  in  microchemical  quali- 
tative analysis  an  amorphous  precipitate  is  the  least  desirable 
form  in  which  a  substance  may  be  separated  for  identification. 
Nevertheless,  it  often  happens  that  such  precipitates  are  obtained 
either  accidentally  or  when  it  is  more  expedient  to  thus  remove 
a  substance  in  order  to  prevent  it  from  interfering  in  subsequent 
testing  for  other  substances. 

In  qualitative  analysis  by  means  of  microscopic  methods  two 
classes  of  amorphous  precipitates  are  met  with:  (a)  Those  which 
require  solution  in  a  special  solvent  from  which  a  crystalline 
compound  eventually  separates,  and  (b)  those  in  which  crystal- 
lization can  be  induced  by  inoculation  with  a  trace  of  the  same 
compound  in  a  crystalline  condition. 

Special  mention  is  here  made  of  the  treatment  of  amorphous 
precipitates  because  in  a  number  of  instances  treatment  with  hot 
concentrated  sulphuric  or  hydrochloric  acids  must  be  resorted 
to  in  order  to  obtain  recognizable  compounds. 


THE  METHODS  OF  MICROCHEMICAL  QUALITATIVE  ANALYSIS    265 

When  a  precipitate  is  to  be  recrystallized  from  hot  concen- 
trated sulphuric  acid,  it  must  be  placed  or  formed  at  the  comer  of 
the  object  slide  and  any  supernatant  aqueous  solution  decanted. 
A  moderate  sized  drop  of  the  concentrated  acid  is  then  placed 
upon  the  precipitate  and  the  slide  immediately  inclined  at  an  angle 
of  at  least  30  degrees,  to  prevent  the  acid  from  spreading.  Heat 
from  a  tiny  flame  is  then  applied  to  the  object  slide  just  below 
the  upper  edge  of  the  drop,  and  as  the  acid  fumes  off  the  flame 
is  brought  nearer  and  nearer  to  the  corner.  As  soon  as  it  appears 
that  sufficient  material  has  passed  into  solution,  the  preparation 
is  removed  from  the  flame  and  allowed  to  cool  for  a  few  seconds, 
while  still  held  in  an  inclined  position.  The  inclined  slide  is  then 
tipped  so  as  to  cause  a  slow  flow  to  the  adjacent  corner  (see  page 
232,  Decantation),  thus  decanting  the  clear  acid  from  the  remain- 
ing insoluble  precipitate,  the  channel  of  flow  is  cut  off  with  filter 
paper  and  the  slide  inclined  until  it  is  almost  vertical,  thus 
causing  the  clear  drop  of  acid  to  gather  at  the  very  corner  of  the 
slide.  This  corner  is  then  touched  to  a  clean  slide  and  through  a 
touch  with  a  glass  rod  or  platinum  wire  the  drop  is  made  to  flow 
from  the  inclined  slide  to  the  horizontal  one.  A  small  clear  drop 
is  thus  obtained. 

This  system  of  attack  can  be  employed  in  all  cases  involving 
re-solution  in  strong  reagents.  Where  constituents  dissolving 
from  the  glass  slide  are  objectionable  platinum  foil  can  be  em- 
ployed, eventually  transferring  as  above  to  a  glass  slide. 

The  second  case  mentioned  arises  most  often  in  the  analysis 
of  organic  compounds,  as,  for  example,  in  the  separation  of  a 
free  base  from  its  salts  by  means  of  an  alkali.  Although  the 
amorphous  appearing  material  will  eventually  crystallize  sponta- 
neously if  given  sufficient  time,  it  is  usually  desirable  to  hasten 
the  formation  of  typical  crystals.  This  can  be  accomplished  by 
taking  upon  a ,  platinum  needle  the  most  minute  fragment 
possible  from  a  portion  of  the  pure  base  believed  to  be  present 
and  drawing  it  through  the  amorphous  mass,  crushing  it  at 
the  same  time.  Crystallization  of  the  amorphous  material  is 
almost  always  immediately  started  and  proceeds  with  great 
rapidity. 


266  ELEMENTARY   CHEMICAL   MICROSCOPY 

EXPERIMENTS. 

a.  Add  (by  Method  /)  to  a  drop  of  BaCL.  solution  a  drop  of  dilute  H2S04, 
evaporate  to  cause  agglutination  of  the  BaS04;  add  a  drop  of  water,  warm 
gently.  Decant.  Recrystallize  the  residue  from  hot  concentrated  H2S04  as  de- 
scribed above.  Cool  and  breathe  repeatedly  upon  the  drop.  Study  the  crystals 
as  they  form. 

b.  Repeat,  using  PbN03  instead  of  BaCl2. 

c.  Precipitate  AgCl  from  a  solution  of  AgN03.  Recrystallize  from  concen- 
trated HC1. 


XIII.  The  material  to  be  analyzed  is  exposed  to  the  action  of 
vapors  or  gases,  or  a  reagent  is  exposed  to  vapors  or  gases  resulting 
from  the  action  of  some  compound  upon  the  material  to  be  tested. 

Oxidation  of  loosely  bound  sulphur  to  sulphate  can  usually  be 
accomplished  by  placing  a  drop  of  bromine  in  a  watch  glass  or 
crucible  (use  the  apparatus,  Fig.  131,  page  246),  inverting  the 
drop  of  a  solution  of  the  substance  to  be  tested  over  the  bromine, 
warming  gently  in  the  hood  and  allowing  the  preparation  to  stand 
for  five  or  ten  minutes  in  contact  with  the  bromine  vapors. 

In  many  instances,  the  substance  need  not  even  be  in  solution, 
but  can  be  merely  in  suspension,  provided  it  is  in  a  finely  divided 
condition.  No  specific  directions  are  necessary  other  than  the 
caution  that  the  inverted  drop  must  never  be  so  large  that  there 
is  danger  of  its  dropping  off  the  object  slide.' 

Never  perform  oxidations  with  bromine  save  in  the  hood  at  a 
distance  from  all  microscopes. 

After  exposure  to  the  oxidizing  vapors,  the  slide  is  removed, 
turned  right  side  up,  the  excess  of  bromine  expelled  in  the  hood 
by  gentle  warming  and  the  remaining  drop  tested  for  the  pres- 
ence of  sulphates. 

In  testing  for  the  presence  of  a  gas,  as,  for  example,  hydrocyanic 
acid,  the  reagent  (in  this  case  silver  nitrate  solution)  may  be  in- 
verted over  the  container  in  which  the  gas  is  liberated,  —  watch 
glass,  crucible  or  test  tube,  --or  in  testing  for  arsenic  through 
the  generation  of  arsine,  the  gases  may  be  conducted  through  a 
tiny  capillary  tube  containing  a  minute  crystal  of  silver  nitrate. 
The  distilling  tube,  Fig.  133,  page  248,  serves  as  an  excellent 
generator  for  applying  this  modification  of  the  Gutzeit  test  for 
arsenic  (see  Figs.  138  and  139). 


THE  METHODS  OF  MICROCHEMICAL  QUALITATIVE  ANALYSIS    267 

In  a  similar  manner  traces  of  moisture  (or  water  of  hydration 
in  tiny  crystals)  can  easily  be  recognized  by  placing  a  minute 
quantity  of  dry  powdered  fuchsine  in  a  capillary  tube  and  causing 
the  moist  air  to  pass  over  it  by  heating.  The  change  from  the 
greenish  black  powder  to  crimson  droplets  is  very  striking. 

Numerous  other  examples  might  be  given. 

EXPERIMENTS. 

a.  Place  in  the  crucible  of  the  apparatus,  Fig.  130,  two  or  three  fibers  of 
asbestos,  drop  upon  them  a  single  drop  of  bromine  (in  the  hood).  Invert  over 
the  crucible  a  drop  of  a  solution  of  a  sulphide.  Lower  the  clamp  and  warm 
gently  in  the  hood,  until  the  crucible  is  filled  with  bromine  vapors.  Allow  to  stand 
for  about  five  minutes.  During  this  period  test  a  portion  of  the  unoxidized  ma- 
terial for  sulphates  as  below.  Lift  off  the  object  slide  from  the  crucible,  turn  it 
drop  side  up  and  evaporate  to  dryness;  add  a  drop  of  water  to  the  cool  residue, 
then  a  tiny  drop  of  HN03.  Decant  if  not  clear,  and  finally  test  for  sulphates  by 
adding  a  drop  of  Ca(C2H302)2.  (Method  /.)  CaS04  •  2H2O  separates  in  the  form 
of  radiating  tufts  or  X's  of  monoclinic  needles  or  thin  prisms. 

b.  Place  in  the  glass  crucible  a  dilute  solution  of  KCN.  Cover  it  with  an 
object  slide,  carrying  a  small  drop  of  AgN03  upon  its  under  side.  Raise  the  slide 
just  enough  to  permit  dropping  in  several  small  grains  of  primary  sodium  car- 
bonate (HNaCOs).  Cover  tightly  at  once  and  allow  to  stand  for  five  or  ten 
minutes.  If,  after  this  interval,  no  cloudiness  is  visible,  warm  the  crucible  gently. 
Remove  the  slide  and  examine  it  with  a  \  inch  or  8  millimeter  objective.  AgCN 
appears  as  small  colorless  prisms  with  obliquely  truncated  ends. 


XIV.  Methods  involving  fusing  the  material  in  a  bead  of  borax, 
microcosmic  salt  or  other  medium. 

Some  of  the  very  earliest  attempts  to  employ  the  microscope 
for  the  detection  of  minute  amounts  of  material  were  made  in 
conjunction  with  the  blowpipe  analysis  of  minerals.  It  was 
found  that  many  substances  yielded  characteristic  crystals 
when  fused  in  borax  beads  before  the  blowpipe  at  high  tem- 
peratures. 

Although  of  questionable  usefulness  in  systematic  analysis, 
this  method  is  of  sufficient  interest  to  the  student  to  be  well 
worthy  of  trial  and  study.1 

To  obtain  a  loop  wind  a  platinum  wire  twice  around  a  glass 

1  See  Sorby,  Chem.  News,  19  (1869),  124;  Wunder,  J.  f.  prak.  Chem.,  109 
(1870),  452;  Emerson,  Proc.  Amer.  Acad.  Arts  and  Sci.,  6,  476. 


268  ELEMENTARY   CHEMICAL   MICROSCOPY 

rod  2  to  4  millimeters  in  diameter.  Heat  the  wire  red  hot,  dip 
into  borax  (or  other  substance)  and  heat  until  a  clear  glassy  bead 
is  obtained  of  from  i  to  2  millimeters  thick.  Cool.  Examine 
under  the  microscope,  using  a  low  power  to  assure  the  absence  of 
crystals.  Heat  and  touch  to  the  powdered  material  to  be  studied. 
Then  very  carefully  heat  the  preparation  in  the  flame  of  a  Bunsen 
burner  until  the  borax  or  phosphorus  salt  bead  just  begins  to  melt. 
Avoid  heating  to  redness.  Cool  and  examine  with  a  16-milli- 
meter objective.  Heat  again,  and  again  place  under  the  micro- 
scope, thus  following  any  changes  which  may  take  place.  Should 
a  blast  lamp  be  employed  for  the  heating  care  must  be  observed 
to  avoid  too  large  and  too  hot  a  flame. 

This  method  can  be  made  to  yield  good  results  in  testing  for 
calcium  and  magnesium  and  also  for  silicon,  zirconium,  titanium 
and  molybdenum.  Colored  bead  reactions  are  also  obtainable, 
as  for  example  in  testing  for  Co,  Ni,  Cr,  Mn,  etc. 

The  general  principle  of  the  method  is,  however,  much  broader 
in  its  scope  since  it  comprehends  all  cases  where  a  crystalline 
phase  will  separate  from  a  transparent  molten  mass  which  solidi- 
fies upon  cooling. 


XV.    Testing  with  Hydrofluoric  Acid  or  Silico fluorides. 

These  reagents  are  applied  in  one  of  the  manners  already  de- 
scribed, usually  by  Methods  I,  7/7,  or  III  A. 

Specific  comment  is  necessary,  however,  because  of  the  im- 
possibility of  employing  ordinary  glass  object  slides  and  because 
of  the  great  danger  of  permanently  damaging  the  objectives 
through  the  corrosive  action  of  hydrofluoric  acid  vapors. 

Before  undertaking  any  tests  in  which  hydrofluoric  acid  vapors 
will  probably  be  present,  remove  all  objectives  from  the  nose- 
piece  save  the  lowest  power,  and  place  all  microscope  accessories 
at  such  a  distance  from  the  preparation  as  to  render  them  safe. 
Take  a  small  cover  glass,  carefully  add  a  tiny  drop  of  pure  glyc- 
erine to  its  center  and  bring  the  drop  in  contact  with  the  lower 
lens  of  the  objective  and  press  gently  until  the  drop  spreads  out 
into  a  thin  film,  holding  the  cover  glass  in  place.     This  is  done  to 


THE  METHODS  OF  MICROCHEMICAL  QUALITATIVE  ANALYSIS    269 

reduce  the  danger  of  corrosion  of  the  lens  by  the  acid  vapor.  If 
a  considerable  period  of  time  is  occupied  in  a  series  of  tests,  the 
cover  glass  should  be  removed  at  intervals  and  the  objective 
thoroughly  wiped  off  and  cleaned  with  lens  paper  moistened  with 
water,  dried  and  a  new  cover  glass  and  glycerine  applied. 

It  is  always  preferable  to  have  a  cheap  objective  set  aside, 
especially  for  hydrofluoric  acid  work,  so  as  not  to  run  the  risk  of 
ruining  an  expensive  lens. 

For  supports  upon  which  to  perform  the  tests,  celluloid  slips 
will  be  found  convenient.  The  chief  difficulty  arises  when  gently 
heating  the  preparation,  to  cause  development  of  the  crystal 
forms,  since  nitrocellulose  is  very  inflammable.  Slips  of  cellulose 
acetate  are  therefore  far  preferable  but  are  at  present  not  com- 
mercially obtainable. 

Glass  object  slides  coated  with  a  film  of  "zapon"  varnish, 
allowed  to  dry,  and  a  second  coat  applied,  yield  good  results  when 
carefully  prepared,  but  require  as  great  care  in  heating  as  cellu- 
loid slips. 

A  better  device  consists  in  coating  glass  object  slides  with 
"Bakelite,"  and  heating  in  an  oven  to  the  temperature  directed 
by  the  Bakelite  Company  for  the  particular  grade  of  "Bakelite" 
used.  Slides  thus  coated  can  be  warmed  without  danger  and 
yield  good  results. 

Whenever  a  critical  case  arises  involving  the  detection  of 
minute  amounts  of  silica,  titanium  or  zirconium,  etc.,  it  is  best 
to  have  recourse  to  cellulose  nitrate  or  acetate  slips  so  as  to  pre- 
clude the  possibility  of  error  due  to  pores  or  fissures  in  the  var- 
nished surface  of  a  glass  slide. 

Decompositions  by  means  of  hydrofluoric  acid  are  best  per- 
formed upon  small  pieces  of  platinum  foil  or  in  the  tiny  platinum 
spoons  shown  in  Fig.  134,  page  248.  Subsequently  the  material 
can  be  transferred  to  cellulose  slips  or  varnished  slides  for 
study. 

In  selecting  slips  made  from  cellulose  compounds,  only  such 
pieces  should  be  chosen  as  are  not  badly  scratched  and  grooved, 
and  which  are  as  nearly  colorless  as  possible.  Deep  yellow  slips 
are  not  suitable  since  in  testing  for  sodium  or  for  silica  we  depend 


270  ELEMENTARY   CHEMICAL   MICROSCOPY 

for  identification  upon  the  faint  pink  tint  of  sodium  silicofiuoride 
as  well  as  upon  its  crystal  form.  The  same  caution  holds  good 
for  "Bakelite"  varnish -- obtain  one  not  highly  colored  if  pos- 
sible and  coat  the  glass  slide  with  only  a  thin  film.  In  coating 
glass  slides  with  any  protective  varnish  always  carry  the  coating 
over  the  edges. 

Glass  slides  varnished  with  Canada  balsam  dissolved  in  chloro- 
form or  xylene  and  subsequently  dried  in  an  oven  at  a  slightly 
higher  temperature  than  that  of  the  room  can  also  be  used,  but 
are  not  so  convenient  as  the  methods  given  above. 

Rathgen  has  recently  called  attention  to  an  entirely  different 
manner  of  employing  fluorides  in  microchemical  reactions.  He 
has  shown  x  that  a  very  sensitive  and  characteristic  reaction  for 
aluminum  may  be  obtained  by  mixing  the  finely  powdered  ma- 
terial with  several  times  its  weight  of  ammonium  fluoride  in  a 
platinum  cup  or  tiny  platinum  crucible,  to  which  is  then  added 
four  or  five  drops  of  sulphuric  acid  and  the  whole  heated  gently 
until  all  volatile  fluorine  compounds  have  been  expelled;  the  heat 
is  next  slowly  raised  to  drive  off  the  sulphuric  acid  and  the  cup 
finally  brought  for  a  moment  to  a  low  red.  After  cooling,  the 
residue  is  transferred  to  an  object  slide  by  means  of  a  drop  of 
water  and  a  tiny  brush.  Aluminum  gives  tiny  six-sided  crystals 
and  hexagonal  plates. 

EXPERIMENTS. 

Experiments  involving  the  use  of  fluorides  will  be  found  outlined  in  Chapter 
XIV  under  the  elements  Sodium,  Barium  and  Silicon. 

PREPARATION   OF   SPECIAL  REAGENTS. 

Ammonium  Mercuric  Sulphocyanate.  —  To  a  concentrated 
solution  of  mercuric  nitrate  add  a  concentrated  solution  of  am- 
monium sulphocyanate  as  long  as  a  precipitate  is  produced. 
Filter;  wash  thoroughly  the  mercuric  sulphocyanate  obtained 
and  transfer  while  wet  to  a  beaker,  and  dissolve  it  by  adding 
slowly  drop  by  drop  a  saturated  solution  of  ammonium  sulpho- 

1  Zeit.  anal.  Chem.,  63  (1914),  33. 


THE  METHODS  OF  MICROCHEMICAL  QUALITATIVE  ANALYSIS    27 1 

cyanate;  as  soon  as  the  precipitate  has  dissolved  add  a  very  slight 
excess  of  the  ammonium  salt.  Too  much  ammonium  sulpho- 
cyanate  decreases  the  sensitiveness  of  the  reagent. 

Litmus-Silk-Fibers.1  —  Boil  a  little  raw  silk  with  water, 
decant  and  wash.  Pour  over  the  silk  fibers  a  solution  of  pure 
litmus.2  Evaporate  on  the  water  bath  to  small  bulk,  but  not  to 
dryness.  Wash  the  colored  silk  with  water  and  dry  by  pressing 
between  filter  paper  and  then  allowing  to  stand  in  the  air  for  a 
time. 

For  red-litmus-silk,  treat  with  the  weakest  possible  acetic  acid 
and  dry  by  pressing  between  filters  just  before  using. 

For  blue-litmus-silk,  treat  with  the  weakest  possible  sodium 
hydroxide  and  dry  between  filters. 

Turmeric-Linen-Fibers.3  —  Boil  commercial  turmeric  root 
with  twice  its  weight  of  alcohol  until  all  the  coloring  matter  is 
extracted.  Filter  and  evaporate  to  dryness  on  the  water  bath. 
Dissolve  the  residue  in  an  amount  of  dilute  alcohol  (to  which  a 
drop  of  sodium  hydroxide  has  been  added)  equal  to  about  half 
the  weight  of  turmeric  root  taken.  Place  the  fibers,  previously 
boiled  in  water,  in  this  solution  and  evaporate  almost  to  dryness 
upon  the  water  bath.  Wash  once  with  water,  press  between 
filter  paper  and  treat  with  water  acidulated  with  sulphuric  acid. 
Press  between  filters,  wash  with  water  and  dry. 

Zinc-Sulphide-Fibers.4  —  Soak  guncotton  in  a  strong  solution 
of  zinc  sulphate,  decant  the  solution  and  pour  upon  the  wet 
cotton  a  dilute  solution  of  sodium  sulphide.  After  standing  a 
short  time,  decant,  wash  the  fibers  with  water  and  dry  them  for 
use. 

Congo-Red-Silk.  —  Prepare  a  strong  solution  of  congo  red, 
make  it  very  slightly  alkaline  and  heat  it  to  boiling;  while  it  is 
still  hot  stir  in  the  coarse  raw  silk  fibers  and  allow  the  prepara- 
tion to  stand  for  a  short  time.  Remove  the  fibers,  wash  very 
thoroughly  with  distilled  water,  press  between  filter  paper  and 

1  Emich,  Monats.,  22  (1901),  670. 

2  Wartha,  Zeit.  anal.  Chem.,  15  (1876),  322. 

3  Donau,  Ann.,  351  (1907),  426. 

4  Donau,  Ann.,  351  (1907),  432. 


272  ELEMENTARY  CHEMICAL  MICROSCOPY 

dry  for  use.  Cotton  fibers  may  be  employed  instead  of  silk, 
but  are  not  so  good.  Mineral  acids,  even  when  in  exceedingly 
dilute  solutions,  turn  the  colored  fibers  blue.  Dilute  organic 
acids  usually  do  not  alter  the  color.  Alkalies  produce  no  change, 
but  will  cause  a  blue  (acid)  fiber  to  turn  red. 


CHAPTER  XIV. 

CHARACTERISTIC  MICROCHEMICAL    REACTIONS  OF  THE 
COMMON    ELEMENTS    WHEN    IN    SIMPLE    MIXTURES. 

The  methods  of  applying  reagents  and  of  performing  the  neces- 
sary manipulations  arising  in  qualitative  analysis  have  already 
been  discussed  at  length  in  Chapter  XIII,  as  well  as  the  applica- 
tion of  the  simple  polarizing  microscope  to  the  differentiation 
of  chemical  compounds  in  Chapter  VIII. 

In  the  directions  which  follow  it  is  assumed  that  the  student 
is  thoroughly  familiar  with  these  topics.  As  an  aid  to  the 
recognition  of  common  salts  which  may  be  met  with,  there  has 
been  given  under  each  element  the  crystal  system  to  which  its 
common  salts  are  to  be  referred.  This  has  been  done  in  the 
hope  that  the  student  will  learn  to  employ  the  polarizing  micro- 
scope and  come  to  appreciate  its  many  advantages  as  an  invalu- 
able aid  and  great  saver  of  time  and  labor.  In  these  tabulations 
the  following  abbreviations  have  been  used:  (I)  Isometric; 
(H)  Hexagonal;  (T)  Tetragonal;  (O)  Orthorhombic;  (M) 
Monoclinic;  (Tr)  Triclinic;  and  the  salts  arranged  in  the  order 
named. 

SODIUM. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 
of  Sodium.1 

A.  ISOTROPIC. 

Isometric.  —  Chlorate.2 

The  alums  (double  sulphates  of  Na  and  Al,  Fe,  Cr) 

(I);   chloride  (I);   bromide  (I);   iodide  (I);3 

molybdate  (I  or  O). 

1  In  the  following  tabulations  the  data  given  have  largely  been  obtained  from 
Groth's  Chemical  Crystallography. 

2  NaC103  although  belonging  to  the  isometric  system  exhibits  circular  polari- 
zation in  crystals.     Its  solution  is  inactive. 

3  Nal  forms  hydrates  optically  active. 

273 


274  ELEMENTARY  CHEMICAL  MICROSCOPY 

B.  ANISOTROPIC. 

Hexagonal.  —  Nitrate  (pseudo  O) ;  normal  phos- 
phate; potassium-sodium  molybdate;  silico- 
fluoride.1 

Tetragonal. 

Orthorhombic.  —  Iodate;  nitrite;  potassium-sodium 
tartrate;  normal  tartrate;  primary  phos- 
phate. 

Monoclinic. — Acetate;  secondary  arsenate;  borates, 
tetra  and  meta;  carbonate;  primary  carbon- 
ate; chromate;  f errocyanide  f  oxalate,  ferric- 
sodium;  secondary  phosphate;  ammonium- 
sodium  acid  phosphate;  sulphate;  primary 
sulphate;  thiosulphate;  zinc-sodium  sul- 
phate. 

Triclinic.  —  Bichromate;  bi tartrate;  primary  oxa- 
late. 

DETECTION. 

A.  —  By  means  of  Uranyl  Acetate. 
Apply  test  by  Method  IV,  page  255. 

Sodium  yields  with  uranyl  acetate  small  "faintly  yellow  tetra- 
hedra,  appearing  black  by  transmitted  light.  The  compound 
formed  probably  has  the  formula  NaCoH302  •  UC^^HsC^o. 
The  crystals  are  isotropic  belonging  to  the  isometric  system. 

Potassium,  rubidium,  cesium  and  ammonium  yield  long 
needles  or  slender  prisms  of  the  tetragonal  system  of  greater 
solubility  than  the  sodium  compound  and  therefore  not  appear- 
ing until  the  preparation  has  evaporated  almost  to  complete 
dryness. 

Because  of  the  high  solubility  of  ammonium  uranyl  acetate, 
SchoorP  has  suggested  its  use  for  detecting  sodium  instead  of 
simple  uranyl  acetate.  The  test  thus  made  is  more  sensitive, 
but  lacks  the  convenience  of  the  method  given  above  in  that  no 

1  Na2SiF6  is  said  to  be  pseudohexagonal. 

2  Na4Fe(CN)6  •  12  H20  is  pseudotetragonal. 

3  Lenz  u.  Schoorl,  Zeit.  anal.  Chem.,  60  (191 1),  263. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS        275 

indication  of  the  probable  presence  of  K,  Rb,  Cs  or  NH4,  can  be 
obtained  at  the  same  time  Na  is  being  searched  for. 

In  the  presence  of  magnesium  there  will  be  obtained  in  addi- 
tion to  the  tetrahedra  of  the  sodium  double  salt  large  monoclinic 
crystals  of  a  triple  salt 

NaC2H302  •  Mg(C2H302)2  •  3  (U02(C2H302)2)  ■  9  H20, 

taking  the  form  of  rhombs  or  appearing  to  be  octahedra,  dodeca- 
hedra  or  having  a  more  or  less  triangular  outline  with  incurving 
sides.  When,  however,  the  amount  of  sodium  is  very  small 
with  reference  to  that  of  magnesium,  only  the  triple  salt  will 
appear. 

As  might  be  expected  any  of  the  other  elements  in  the  magne- 
sium group  in  the  Periodic  System,  Gl,  Zn,  Cd,  can  replace  Mg 
in  the  triple  salt. 

Precautions. 

Too  much  free  acid  interferes  with  the  test  —  a  further  reason 
for  evaporation  to  dryness  before  applying  the  reagent. 

Much  magnesium  gives  rise  to  a  film  of  salts  so  hygroscopic 
that  a  dry  film  cannot  be  obtained  unless  the  salts  are  first 
converted  into  sulphates  by  evaporation  with  a  little  dilute 
sulphuric  acid. 

Members  of  the  calcium  group  often  cause  trouble.  If,  there- 
fore, an  unsatisfactory  test  for  sodium  is  obtained  and  subse- 
quent testing  reveals  the  presence  of  Ca,  Sr  or  Ba,  these  ele- 
ments should  be  removed  by  precipitation  with  sulphuric  acid, 
the  solution  filtered  or  decanted  from  the  precipitate  and  the 
filtrate  evaporated  to  dryness  on  platinum  (why  ?)  and  again 
tested  for  sodium. 

Any  compounds  present  in  the  material  to  be  tested  which 
will  yield  an  insoluble  precipitate  with  uranyl  acetate,  as,  for 
example,  phosphates,  will  naturally  seriously  interfere  with  the 
test  or  may  absolutely  prevent  the  detection  of  Na.  In  such  an 
event  the  amount  of  uranyl  acetate  employed  must  be  slightly 
more  than  sufficient  to  satisfy  all  the  P04  present  and  to  unite 
with  the  sodium  to  form  the  double  salt.  Under  these  condi- 
tions this  test  becomes  unsatisfactory  as  applied  above  since  it 


276  ELEMENTARY   CHEMICAL   MICROSCOPY 

requires  too  much  time.  It  is  then  better  to  flood  the  dry  film 
with  reagent,  allow  a  few  seconds  to  elapse  for  the  establish- 
ment of  equilibrium  and  decant  the  clear  solution  from  the  pre- 
cipitate of  uranyl  phosphate.  The  decanted  solution  must  then 
be  allowed  to  evaporate  spontaneously  until  crystallization  sets 
in,  or  the  evaporation  may  be  hastened  by  gentle  heating. 

This  test  for  sodium  is  also  apt  to  prove  unsatisfactory  in  the 
presence  of  much  potassium.  To  remove  the  latter  add  per- 
chloric acid  in  slight  excess.  Evaporate  to  dryness,  moisten  the 
residue  with  perchloric  acid  and  again  evaporate.  Extract  the 
residue  with  alcohol;  potassium  perchlorate  is  insoluble;  so- 
dium perchlorate  passes  into  solution  (Schoorl).  Evaporate  the 
clear  alcoholic  extract  to  dryness  and  test  for  sodium. 

A  further  caution  is  necessary  relative  to  the  possible  inter- 
ference of  elements  such  as  Fe,  Mn,  Ni  and  Co,  which  can  form 
double  acetates  with  uranyl  acetate  and  thus  reduce  the  amount 
of  the  reagent  available  to  form  the  double  sodium  compound. 

EXPERIMENTS. 

Test  for  Xa  in 

a.  NaCl,  Na2S04,  HNa2P04. 

b.  NaKC^jOe;  and  in  3(Na2C204)  •  Fe2(C204)3. 

c.  A  mixture  of  NaCl  and  MgS04  and  of  NaCl  and  MgCl2. 

d.  A  mixture  of  Na2S04  and  ZnS04. 


B.   By  means  of  Bismuth  Sulphate. 

First  convert  the  compound  to  sulphate  by  evaporations 
to  dryness  with  sulphuric  acid. 

Apply  the  bismuth  sulphate  by  Method  77,  page  252. 

Immediately  after  the  addition  of  the  unknown  to  the  reagent, 
gently  warm  the  preparation  over  the  micro-burner. 

Sodium  bismuth  sulphate  3Na2S04  •  2Bi2(S04)3  separates  in  the 
form  of  colorless  slender  rods  or  prisms  with  almost  rounded 
ends,  uniting  in  crosses,  X's,  or  more  or  less  star-like  radiating 
clumps.  The  crystals  separating  near  the  circumference  of  the 
drop  are  usually  shorter,  stouter  and  more  prismatic,  while  those 
nearer  the  center  are  more  rod-like.  It  is  these  rod-like  crystals 
with  parallel  extinction  which  are  the  more  characteristic  and 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS        277 

unless  these  are  obtained  the  conclusion  that  sodium  is  present 
is  unwarranted. 

Potassium  sulphate  yields  plates  having  a  hexagonal  or  coffin- 
like  outline  or  six-pointed  stars  and  rosettes.  When  first  formed 
these  plates  appear  as  circular  disks  but  they  rapidly  acquire  six 
sides  or  grow  into  rosettes.  Ammonium,  rubidium  and  cesium 
form  similar  hexagons  and  rosettes. 

When  both  sodium  and  potassium  are  present,  the  rod-like 
crystals  of  the  sodium  double  salt  and  the  hexagons  of  the  potas- 
sium salt  each  appear,  permitting  a  simultaneous  detection  of 
sodium  and  potassium. 

The  addition  of  a  very  minute  quantity  of  glycerine  to  the 
preparation  before  heating  usually  yields  better  crystals  and 
more  reliable  results. 

Precautions. 

Tufts  of  fine  radiating  needles  appearing  greyish  or  brownish 
by  transmitted  light  must  not  be  regarded  as  indicating  the 
presence  of  sodium;  neither  should  stout  prisms  or  elongated 
plates  with  forked  or  broken  ends. 

It  is  always  best  to  remove  members  of  the  calcium  group  by 
means  of  sulphuric  acid  before  applying  the  bismuth  sulphate 
test.  Calcium  is  especially  to  be  guarded  against  since  calcium 
sulphate  may  assume  forms  which  simulate  tlje  sodium  double 
salt ;  for  although  the  crystals  CaS04  •  2  H20  are  monoclinic  and 
usually  lie  in  positions  yielding  oblique  extinction,  the  extinction 
angle  is  small  and  unless  care  is  exercised  the  student  may  credit 
them  with  parallel  extinction. 

Free  mineral  acids  (especially  nitric)  greatly  retard  the  sepa- 
ration of  sodium  bismuth  sulphate. 

In  the  absence  of  bismuth  sulphate  the  reagent  may  be  pre- 
pared as  follows:  At  the  corner  of  a  slide  place  a  drop  of  dilute 
sulphuric  acid;  add  to  this  drop  a  little  basic  bismuth  nitrate 
and  stir  until  the  bismuth  salt  has  completely  dissolved.  Heat 
carefully  until  the  water  has  been  mostly  expelled,  and  crystal- 
lization of  the  bismuth  sulphate  takes  place;  then  add  a  rather 
large  drop  of  water  and  a  very  minute  drop  of  dilute  nitric  acid. 


278  ELEMENTARY   CHEMICAL  MICROSCOPY 

Stir  for  a  few  moments.  The  reagent  drop  should  now  slowly 
clear  up,  and  a  perfectly  clear  solution  should  result.  If,  how- 
ever, the  quantity  of  bismuth  nitrate  employed  has  been  exces- 
sive, a  residue  remains;  it  is  then  necessary  to  decant  the  clear 
liquid.  On  another  slide,  or  better  on  platinum  foil,  heat  with 
dilute  sulphuric  acid  a  few  particles  of  the  substance  to  be 
tested.  Drive  off  the  excess  of  acid;  cool  and  stir  to  provoke 
crystallization.  If  the  drop  refuses  to  crystallize,  add  more  of 
the  substance  and  heat  again.  A  drop  of  the  reagent  prepared 
as  above  is  placed  at  the  corner  of  a  slide,  and  to  it  is  added,  at 
the  center,  without  stirring,  a  little  of  the  moist  mass  of  the 
material  to  be  tested,  taken  from  the  platinum  foil.  Warm  the 
preparation  gently  by  holding  it  for  a  second  or  two  about  one 
centimeter  above  the  micro-flame.  Cool  rapidly  and  examine 
at  once. 

This  reaction  is  more  valuable  for  potassium  than  for  sodium 
and  constitutes  one  of  the  best  microchemical  tests  for  bismuth. 

EXPERIMENTS. 

"Test  for  Na  in  NaCl;  HNa2P04;  in  mixture  of  salts  of  Na  and  K  and  in  mix- 
tures of  salts  of  Na  and  Ca. 


C.   By  Means  of  Ammonium  Silicofluoride. 

See  precautions  given  under  Method  XV,  page  268. 

To  the  drop  of  the  neutral,  or  at  the  most  only  slightly  acid 
solution  of  the  material  to  be  tested,  add  a  fragment  of  ammo- 
nium silicofluoride.  Allow  to  stand  some  time  (but  never  upon  the 
stage  of  the  microscope)  or  hasten  the  reaction  by  gentle  warming. 

Sodium  silicofluoride  Na2SiF6  separates  in  the  form  of  six- 
sided  plates  or  prisms  belonging  to  the  hexagonal  (?)  system. 
Unless  the  crystals  are  excessively  thin  they  appear  with  trans- 
mitted light  to  have  a  very  faint  rosy  tint.  They  polarize  only 
feebly. 

The  corresponding  potassium  salt  of  like  formula  is  much 
more  soluble,  separates  only  from  decidedly  concentrated  solu- 
tions, and  crystallizes  in  small,  colorless  cubes,  octahedra  and 
combinations  of  these  two,  or  in  dodecahedra  (isometric).     A 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS        279 

hexagonal  or  pseudo-hexagonal  modification  of  potassium  sili- 
cofluoride  is  also  known  but  is  formed  only  at  low  temperatures. 
There  is  no  possible  danger,  therefore,  of  confusing  sodium  and 
potassium.  It  is  well  to  remember,  however,  that  undue  de- 
velopment of  the  diagonally  opposite  faces  of  an  octahedron 
yields  a  crystal  giving  an  image  hexagonal  in  outline.  The 
color  of  the  crystal  and  its  action  on  polarized  light  should  leave 
no  room  for  doubt  as  to  its  identity. 

From  very  concentrated  solutions,  in  addition  to  potassium, 
Li,  Ca,  Sr,  Mg,  Mn,  Fe,  etc.,  may  possibly  separate. 

Barium,  if  present,  is  always  precipitated  with  sodium,  form- 
ing barium  silicofluoride  BaSiF6,  which  cannot  be  confused  with 
the  sodium  salt  since  the  barium  compound  crystallizes  in 
rods  or  fusiform  crystals  singly,  in  crosses  or  in  irregular  masses. 
Neither  calcium  nor  strontium  are  precipitated  by  ammonium 
silicofluoride,  but  each  salt  is  liable  to  separate  from  too  concen- 
trated solutions.  The  calcium  salt  CaSiF6  •  2  H20  (monoclinic) 
forms  spindle-shaped  crystals,  and  though  these  are  grouped  in 
rosette-like  masses,  they  are  not  to  be  mistaken  for  sodium. 

The  magnesium  salt  MgSiF6  •  6  F£20  is  so  much  more  soluble 
than  those  above  mentioned  as  to  never  separate  save  upon 
evaporation  or  from  very  concentrated  solution.  Its  crystals 
are  rhombohedra,  polarize  strongly  and  do  not  have  a  six-sided 
outline.  The  silicofluoride  of  iron  is  isomorphous  with  the 
magnesium  salt. 

It  is  evident  that  if  silicon  is  present  in  the  material  under 
examination,  we  can  test  for  sodium  and  silicon  in  one  operation 
by  adding  ammonium  fluoride  and  then  acidifying.  A  pre- 
cipitation of  crystals  resembling  sodium  silicofluoride  would 
point  to  the  presence  of  sodium  and  silicon,  or  an  element  be- 
having, under  like  conditions,  similarly  to  silicon.  Thus  we 
have  titanofluorides,  zirconofluorides  and  stanofluorides  from 
elements  of  the  fourth  group;  and  from  the  transitional  ele- 
ments, glucinum  in  the  second  group  and  boron  in  the  third, 
we  may  have  glucinofluorides  and  borofluorides  of  sodium.  Of 
these  compounds  the  titanofluoride  is  known  to  be  isomorphous 
with  the  silicofluoride  of  sodium. 


2So  ELEMENTARY   CHEMICAL   MICROSCOPY 

In  the  absence  of  ammonium  silicofluoride,  pure  silicon  dioxide 
and  ammonium  fluoride  can  be  added  to  the  acidified  drop  of 
the  solution  to  be  examined. 

Precautions. 

Neither  ammonium  silicofluoride  nor  ammonium  fluoride 
should  ever  be  employed  without  having  first  been  tested  for  the 
presence  of  sodium.  If  the  reagents  are  found  to  be  impure,  it 
is  necessary  to  sublime  them  in  a  platinum  crucible,  or  receive 
the  sublimate  on  platinum  foil  held  over  the  material  heated  in 
a  platinum  cup. 

In  the  presence  of  much  calcium  the  crystals  of  sodium  silico- 
fluoride may  become  distinct  hexagonal  prisms  instead  of  hexag- 
onal plates,  a  fact  which  must  be  borne  in  mind  when  working 
with  material  of  unknown  composition. 

The  silicofluoride  test  is  one  of  the  most  valuable  at  our  com- 
mand in  testing  silicates  for  sodium,  in  which  case  we  need  add 
only  hydrofluoric  acid  or  ammonium  fluoride  and  sulphuric  acid. 

The  addition  of  sodium  and  a  fluoride  gives  us  a  test  for  Si, 
Ti  or  B. 

Remember  that  glass  slides  cannot  be  used  in  this  test  for 
sodium;  that  only  low-power  (i  inch)  objectives  of  great  work- 
ing distance  should  be  employed,  and  even  then  the  front  lens 
should  always  be  protected  in  some  way,  as,  for  example,  with  a 
small  cover  glass  held  in  place  with  glycerine,  oil  or  other  suitable 
substance.  The  preparation  should  be  examined  as  rapidly  as 
possible,  and  must  be  quickly  removed  from  the  stage.  When 
the  microscope  is  provided  with  a  nosepiece,  it  is  advisable  to 
remove  the  objectives  not  in  use  before  examining  any  prepa- 
rations liable  to  give  off  hydrofluoric  acid  or  volatile  fluorine 
compounds.  The  objective  must  always  be  thoroughly  cleaned 
after  any  such  tests. 

EXPERIMENTS. 

a.  Test,  as  directed  above,  salts  of  Na  in  both  neutral  and  acid  solutions. 

b.  In  order  to  better  appreciate  the  reasons  for  employing  celluloid  slips,  place 
a  drop  of  water  on  a  glass  slide,  acidulate  (but  add  no  Xa),  then  add  the  reagents 
and  examine  the  preparation. 

c.  Try  to  obtain  crystals  of  K2SiF6  from  KC1. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       28 1 

d.  Add  a  little  CaCl2  to  a  solution  containing  Na  and  test  as  above. 

e.  To  a  solution  of  NaCl  add  a  little  Si02  or  a  trace  of  sodium  silicate,  then 
add  NH4F  and  an  acid. 

/.   Repeat  using  some  Ti  compound  in  place  of  that  of  Si. 
g.    Test  a  salt  of  Ba  as  above,  then  a  mixture  of  Ba  and  Na.     Note  that  it 
constitutes  an  excellent  test  for  Ba  even  in  the  presence  of  Na. 


POTASSIUM. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 
of  Potassium. 

A.  ISOTROPIC. 

The  alums  (I);  chloride  (I);  bromide  (I);  iodide 
(I);  cyanide  (I);  molybdate  (I);  silicofluor- 
ide  (I  or  H). 

B.  ANISOTROPIC. 

Hexagonal.  —  Barium-potassium  f errocyanide ;  bo- 
rate, tetra;  silicorluoride  (H  or  I). 

Tetragonal. — Arsenate;  cyanate;  secondary  phos- 
phate. 

Orthorhombic.  — Antimonyl  tartrate;  chlorate;  chro- 
mate;  nitrate;  perchlorate;  permanganate; 
sulphate;  primary  sulphate;  sulphocyanate ; 
primary  tartrate;  sodium-potassium  tartrate. 

Monoclinic.  —  Carbonate;  chlorate;  ferricyanide; 
ferrocyanide ;  iodate;  oxalates;  normal  tar- 
trate. 

Triclinic.  —  Bichromate;   persulphate. 

DETECTION. 

A.   By  Means  of  Chloroplatinic  Acid. 

Apply  the  reagent  by  Method  /,  page  251. 
In  a  few  moments,  relatively  large  and  beautifully  formed, 
strongly  refractive,  bright,  deep  yellow  crystals  of  K2PtCl6 
appear.  The  usual  form  is  that  of  the  regular  octahedron,  some- 
times showing  faces  of  the  cube.  Horizontally  elongated  octa- 
hedra,  or  octahedra  shortened  parallel  to  one  of  the  pairs  of 
faces,  are  not  unusual. 


282  ELEMENTARY  CHEMICAL  MICROSCOPY 

Since  the  crystals  usually  lie  on  one  of  the  faces  of  the  octa- 
hedron, there  is  apt  to  result  an  abnormal  development  of  this 
face  and  the  diagonally  opposite  and  parallel  face;  the  resulting 
crystal  will  thus  exhibit  an  hexagonal  outline  when  seen  through 
the  microscope,  i.e.,  viewed  from  above.  Combinations  of  cube 
and  octahedron  may  lead  to  a  somewhat  similar  appearance. 

Not  infrequently  preparations  are  obtained  in  which  twinning 
is  very  marked,  and  others  in  which  there  is  a  grouping  of  crys- 
tals in  threes  or  fours.  Of  the  twin  crystals,  one  form  seems  to 
predominate;  it  results  from  the  union,  in  reversed  position,  of 
two  halves  of  an  octahedron  where  the  dividing  plane  is  parallel 
to  the  two  opposite  faces. 

The  size  and  rate  of  development  of  the  crystals  formed  will 
depend  largely  upon  the  concentration  of  the  test  drop.  In 
very  concentrated  solutions,  minute  crystalline  grains  or  the 
skeletons  of  octahedra  are  produced.  In  very  dilute  solutions 
the  crystals  appear  only  after  some  time.  In  case  the  test  drop 
proves  to  be  of  the  latter  sort,  heat  it  gently  to  cause  slight 
evaporation,  or  expose  to  alcohol  vapor,  see  Method  VI,  page 
257- 

Thin  crystals  are  lemon  yellow  in  color,  but  those  which  attain 
a  considerable  thickness  are  of  a  decided  orange  tint. 

The  best  results  are  obtained  from  neutral  solutions  or  those 
which  are  very  slightly  acid  with  hydrochloric  acid.  Excess  of 
mineral  acids  is  to  be  avoided,  sulphuric  acid  in  particular. 
Either  evaporate  and  remove  them,  or  mitigate  their  action  by 
adding  sodium  acetate  or  sodium  carbonate.  If  the  latter  salt 
is  used,  care  should  be  taken  to  avoid  making  an  alkaline  solu- 
tion and  a  large  excess  of  the  chloroplatinic  acid  must  always 
be  used. 

Ammonium,  rubidium,  cesium  and  thallous-thallium  also  give 
octahedral  crystals  with  chloroplatinic  acid,  the  composition  of 
the  salts  being  similar  to  that  of  the  potassium  salt.  The 
solubility  of  these  compounds,  and  consequently  the  size  of 
the  crystals  produced,  decreases  rapidly  in  the  order  in  which 
the  elements  are  named.  Ammonium  will  give  octahedra  of  the 
same  size  as  those  of   potassium,   hence  its  absence  must  be 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS        283 

assured  before  the  test  can  be  considered  conclusive  of  the  pres- 
ence of  potassium. 

Salts  of  sodium  form  sodium  chloroplatinate  Na2PtCl6  •  6  ELO, 
a  quite  soluble  salt  crystallizing  in  yellow  triclinic  prisms,  having 
an  extinction  angle  of  about  22  degrees,  and  usually  exhibiting 
brilliant  polarization  colors.  It  is  seldom  that  well-formed, 
distinct  crystals  can  be  obtained,  the  result  generally  being  an 
aggregate  of  imperfectly  developed  crystals.  The  salt  is  soluble 
in  even  strong  alcohol,  so  that  the  addition  of  this  reagent  will 
not  cause  the  separation  of  crystals,  but  evaporation  is  hastened. 

The  chloroplatinates  of  potassium,  rubidium,  cesium  and  am- 
monium are  isometric.  That  of  glucinum,  which  is  also  obtained 
when  evaporation  is  practiced,  is  tetragonal.  Lithium  forms  a 
very  soluble  chloroplatinate  similar  to  that  of  sodium. 

Precautions. 

If  salts  of  ammonium  are  present,  or  suspected  of  being 
present,  place  a  little  of  the  material  to  be  tested  on  platinum 
foil,  moisten  with  water,  dry  and  ignite  carefully,  until  all  the  am- 
monium salts  have  been  driven  off.  Dissolve  a  portion  of  the 
residue  in  water,  with  the  addition  of  a  little  hydrochloric  acid 
if  necessary;  transfer  to  a  glass  slide,  and  test;  then  again  ignite 
the  remainder  of  the  residue  and  test  again. 

The  reagent  should  never  be  employed,  even  though  freshly 
prepared,  without  first  testing  it  by  evaporation  to  ascertain 
whether  octahedral  crystals  are  deposited,  since  potassium  may 
have  been  extracted  from  the  containing  vessel,  or  ammonium 
absorbed  from  the  air.  In  making  the  reagent  from  metallic 
platinum  it  must  be  borne  in  mind  that  the  acids  employed  may 
contain  salts  of  potassium  or  ammonium,  or  both. 

When  the  potassium  salt  consists  of  a  compound  other  than 
the  chloride  it  is  always  best  to  evaporate  repeatedly  with  strong 
hydrochloric  acid  before  applying  the  platinum  reagent. 

EXPERIMENTS. 

a.  Test  as  above  KC1,  NaCl,  NH4CI. 

b.  Test  a  phosphate,  a  sulphate,  and  a  tartrate  of  potassium. 

c.  Test  K2S04  in  the  presence  of  much  H2S04. 


284  ELEMENTARY  CHEMICAL  MICROSCOPY 

B.   By  Means  of  Bismuth  Sulphate. 

For  method  of  applying  the  test  and  discussion  of  the  prop- 
erties of  the  salt  formed  see  Test  B  under  Sodium,  page  276. 

Potassium  bismuth  sulphate  3  K2S04  •  Bi2(S04)3  separates  first 
as  circular  disks  which  later  develop  into  hexagonal  plates  or  the 
skeletons  of  hexagons,  i.e.,  six-pointed  stars  and  rosettes. 

Ammonium  salts  yield  similar  crystals.  Hence  this  test  can- 
not be  used  to  differentiate  between  potassium  and  ammonium. 
Precautions. 

See  Sodium,  Method  B. 

EXPERIMENTS. 

See  Sodium,  Method  B. 


C.   By  Means  of  Perchloric  Acid. 
Apply  the  reagent  by  Method  /,  page  251. 

In  a  few  seconds,  colorless,  highly  refractive,  clear-cut  crys- 
tals of  potassium  perchlorate  KC104  separate.  These  crystals 
belong  to  the  orthorhombic  system,  but  at  first  sight  those  first 
formed  usually  appear  to  be  isometric,  while  later,  forms  which 
might  be  mistaken  for  monoclinic  prisms  appear. 

Rubidium  and  cesium  give  a  like  reaction,  and  their  per- 
chlorates  are  more  insoluble  than  that  of  potassium.  Thallium 
forms  an  even  more  insoluble  perchlorate.  The  perchlorates 
of  the  elements  of  the  other  groups  that  are  generally  met  with 
in  ordinary  work,  are  sufficiently  soluble  not  to  interfere. 

Potassium,  rubidium,  and  cesium  perchlorates  possess  a  re- 
markable adsorptive  power  for  potassium  permanganate.  The 
crystals  are  not  altered  in  habit,  size  or  rapidity  of  formation  but 
become  colored  rose  or  rose-violet.  The  compounds  resulting 
are  a  solid  solution  of  potassium  permanganate  in  the  per- 
chlorates and  are  considered  by  crystallographers  to  be  iso- 
morphous  mixtures  of  the  two  salts. 

Advantage  may  be  taken  of  this  property  of  the  potassium 
salt  to  obtain  an  exceedingly  beautiful  test,  for  if  the  test  drop 
contains  sodium  permanganate,  the  potassium  perchlorate  sepa- 
rating therefrom  will  be  colored.     Add  to  the  test  drop  a  little 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       285 

sodium  manganate,1  so  as  to  impart  a  distinct  green,  then  add 
a  tiny  drop  of  hydrochloric  acid,  thus  converting  the  manganate 
into  permanganate.  The  perchloric  acid  is  then  caused  to  flow 
in.  The  crystals  of  potassium  perchlorate  which  separate  have 
the  same  form  as  before,  but  are  a  beautiful  deep  rose  color,  the 
color  intensity  varying  with  the  amount  of  permanganate  present. 
In  a  few  moments  the  liquid  is  completely  decolorized,  and  the 
precipitated  crystals  deeply  colored.  Performed  in  this  way  the 
test  is  a  most  interesting  and  instructive  one. 

The  perchlorate  reaction  is  of  more  value  for  the  detection  of 
the  acid  by  means  of  rubidium  chloride  and  for  the  removal  of 
potassium  to  prevent  interferences  with  tests  for  other  elements, 
than  for  the  identification  of  potassium. 

Precautions. 

To  obtain  truly  satisfactory  results,  careful  attention  to  con- 
centrations must  be  given,  for  if  the  solution  is  too  concentrated 
potassium  perchlorate  is  precipitated  at  once  in  malformed  or 
skeleton  crystals;  while  if  too  dilute  the  separation  of  the  solid 
phase  is  too  slow. 

Exposure  to  alcohol  vapor  hastens  the  reaction. 

In  the  absence  of  perchloric  acid  ammonium  perchlorate  may 
be  used. 

EXPERIMENTS. 

a.  Try  the  above  reaction  with  different  salts  of  K. 

b.  Introduce  NaMnO*  into  the  test  drop,  and  test  as  above. 

c.  Make  a  mixture  of  K  and  Na  salts.  Treat  a  drop  of  a  solution  of  this  mate- 
rial with  HCIO4,  evaporate,  treat  with  the  reagent  again  and  again  evaporate, 
extract  the  dry  residue  with  alcohol,  and  test  the  alcoholic  extract  for  sodium  with 

U02(C2H30,)2. 

d.  Try  the  action  of  HC104  on  members  of  the  magnesium  group,  and  upon 
members  of  the  calcium  group. 


AMMONIUM. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 
of  Ammonium. 

1  Sodium  manganate  is  employed  instead  of  sodium  permanganate  because  it 
is  more  stable  as  a  laboratory  reagent. 


286  ELEMENTARY   CHEMICAL   MICROSCOPY 

A.  ISOTROPIC. 

The  alums  (I) ;  chloride  (I) ;  bromide  (I) ;  iodide 
(I);  silicofluoride  (I). 

B.  ANISOTROPIC. 

Hexagonal.  —  Fluoride. 

Tetragonal.  —  Borate  (NtL^I^Oy  •  4  H20;  primary 
phosphate. 

Orthorhombic. —  Bicarbonate;  nitrate;1  primary 
oxalate;  normal  oxalate;  perchlorate;  pri- 
mary tartrate;  sulphate. 

Monoclinic.  —  Secondary  arsenate;  bichromate; 
chromate;  molybdate;  persulphate;  ammo- 
nium-sodium acid-phosphate;  secondary 
phosphate;  primary  sulphate;  ammonium- 
ferrous  sulphate;  sulphocyanate;  normal 
tartrate;  thiosulphate. 

Triclinic. 

DETECTION. 

Unless  the  analyst  is  dealing  with  a  simple  salt  of  ammonium, 
it  is  always  best  to  expel  the  NH3  from  the  compound  by  distilla- 
tion (see  page  244)  with  sodium  hydroxide  or  magnesium  oxide. 
The  ammonia  set  free  is  fixed  by  absorption  in  a  drop  of  dilute 
hydrochloric  acid  (or  other  acid).  The  resulting  solution  of 
ammonium  chloride  is  concentrated  or  evaporated  to  dryness 
and  the  material  thus  obtained  tested  for  ammonium. 

A.   By  Means  of  Chloroplatinic  Acid. 

See  Method  I,  page  251,  and  discussion  and  precautions 
given  under  Potassium,  test  A,  page  281. 


B.    Through  the  Formation  of  Ammonium  Magnesium  Phos- 
phate. 

The  typical  reaction  for  this  identity  test  may  be  written 

NH4CI  +  MgCl2  +  HNa2P04  +  NaOH  = 
NH4MgP04  +  3  NaCl  +  H20. 

1  NH4NO3  is  pseudotetragonal. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS        287 

To  the  drop  to  be  tested  add  a  fragment  of  sodium  phosphate 
and  a  very  little  magnesium  chloride,  stir  thoroughly.  Beside  the 
drop  place  a  drop  of  dilute  solution  of  sodium  hydroxide  and 
cause  this  drop  to  flow  into  the  other. 

Ammonium  magnesium  phosphate  separates  in  crystals  having 
the  formula  NHjMgPC^  •  6  H20,  belonging  to  the  orthorhombic 
system  and  exhibiting  an  exceptionally  strong  tendency  to  assume 
hemihedral,  hemimorphic  and  skeletal  forms.  This  compound 
usually  separates  first  as  an  almost  amorphous  precipitate  which 
soon  changes  into  star-like  and  X-shaped  crystallites.  Soon  the 
X's  fill  out  and  envelope-like  crystals  result  and  at  the  same  time 
rectangular  prisms  resembling  roofs  of  houses  appear. 

In  preparations  containing  but  little  of  the  ammonium  mag- 
nesium phosphate  the  stars  and  X's  are  usually  absent. 

Precautions. 

Since  the  amount  of  ammonia  obtained  upon  distillation  is 
usually  small  it  is  quite  necessary  to  avoid  an  excess  of  the  mag- 
nesium salt  and  also  the  phosphate,  for  the  reason  that  magne- 
sium phosphate  is  almost  sure  to  be  precipitated.  This  latter 
salt  appears  as  an  amorphous  deposit  and  if  conditions  are  favor- 
able it  may  eventually  crystallize  in  star-like  crystal  aggregates, 
distinct,  it  is  true,  from  the  ammonium  magnesium  phosphate, 
yet  very  apt  to  confuse  the  beginner. 

If  the  phosphate  test  be  applied  directly  to  a  solution  of  the 
unknown  salt  it  must  be  remembered  that  both  phosphates  and 
hydroxides  of  a  number  of  elements  will  probably  be  precip- 
itated. 

EXPERIMENTS. 

Test  as  above  for  the  presence  of  NFL.  in  several  different  salts  containing  this 
radical. 

CALCIUM. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 
of  Calcium. 

A.   ISOTROPIC. 


288  ELEMENTARY   CHEMICAL   MICROSCOPY 

B.   ANISOTROPIC. 

Hexagonal.  —  Carbonate  (H  or  0) ;   chloride. 

Tetragonal.  —  Oxalate. 

Orthorhombic.  -  -  Arsenate    (O   or   M) ;     chromate 

(O  or  M);  tartrate. 
Monoclinic.  --  Nitrate;  sulphate;  double  sulphates 

of  calcium  and  sodium  or  potassium. 
Triclinic.  —  Ferrocyanide. 

DETECTION. 

A.   By  Means  of  Dilute  Sulphuric  Acid. 
Apply  the  reagent  by  Method  /,  page  251. 

If  calcium  is  present,  monoclinic  crystals  of  calcium  sulphate 
will  rapidly  appear  near  the  circumference  of  the  drop  of  the 
substance.  These  crystals  take  the  form  of  exceedingly  slender, 
colorless,  transparent  needles,  either  singly,  in  sheaves,  in 
bundles  or  in  star-like  clusters.  When  in  tiny  sheaves  near  the 
edge  of  the  drop  the  crystals  have  often  a  more  or  less  brownish 
tint  when  seen  by  transmitted  light.  Shortly  after  the  appear- 
ance of  the  bunches  of  needles  at  the  periphery,  long,  thin, 
slender  and  plate-like  prisms  with  obliquely  truncated  ends  are 
formed  throughout  the  drop.  These  prisms  are  frequently 
twinned,  yielding  so-called  arrowhead  or  swallow-tailed  and 
X-like  twins.  These  twin  crystals  are  the  most  characteristic 
of  the  forms  assumed  by  calcium  sulphate  of  the  formula 
CaS04  •  2  H20. 

If  no  crystals  are  visible  after  waiting  a  short  time,  the  prepa- 
ration may  be  cautiously  concentrated.  This  procedure  (evapo- 
ration) may,  however,  lead  to  the  separation  of  such  an  amount 
of  other  salts  as  to  render  difficult  the  detection  of  the  crystals 
of  calcium  sulphate.  A  better  plan  is  to  hasten  the  separation  of 
the  calcium  salt  by  exposing  the  test  drop  to  the  vapor  of 
alcohol;  see  page  257,  Method  VI. 

Salts  of  strontium  may,  under  exceptional  conditions  (if  the 
preparation  be  examined  at  once),  yield  a  precipitate  which 
closely  resembles  that  given  by  calcium.  These  crystals  of 
strontium   sulphate   rapidly   disintegrate,    however,    and   there 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS        289 

results  a  fine  granular  deposit.  This  granular  or  sandy  deposit 
is  the  form  assumed  by  strontium  sulphate  under  the  conditions 
which  ordinarily  obtain  in  this  test.  Barium  is  immediately  pre- 
cipitated in  an  exceedingly  finely  divided  condition,  amorphous 
in  appearance,  but  occasionally  BaS04  separates  in  crystalline 
form  (see  Barium). 

Any  lead  which  may  be  present  will  also  be  precipitated  as  a 
dense  white  amorphous  powder.  Occasionally,  however,  lead 
will  yield  a  precipitate  consisting  of  orthorhombic  crystals. 

Silver  will  separate  as  Ag2S04  in  the  form  of  colorless,  highly 
refractive,  orthorhombic  prisms,  rhombs  or  crystallites  of  char- 
acteristic appearance. 

When  the  drop  of  sulphuric  acid  flows  into  the  drop  to  be 
tested  which  contains  mercurous  nitrate  or  other  soluble  mercu- 
rous  salts,  the  mercurous  sulphate  produced  often  assumes  at 
first  the  form  of  acicular  needles,  closely  resembling  those  of 
calcium  sulphate;  they  are,  however,  blackish  by  transmitted 
light  and  rapidly  take  the  shape  of  rod-like  prisms  quite  distinct 
from  the  prismatic  forms  of  the  calcium  salt. 

Precautions. 

Before  applying  the  sulphate  test,  add  a  drop  of  dilute  hydro- 
chloric acid  to  assure  the  absence  of  lead,  silver  and  mercurous 
salts.     If  a  precipitate  is  formed  decant. 

It  is  not  always  wise  to  conclude  that  calcium  is  present  when 
crystals,  which  apparently  resemble  the  star-  and  sheaf-like  ag- 
gregates of  calcium  sulphate,  separate  at  once  on  the  addition 
of  sulphuric  acid,  even  if  the  crystals  exhibit  oblique  extinction. 
It  sometimes  happens  that  other  compounds,  not  calcium  sul- 
phate, separate  in  forms  not  to  be  distinguished,  at  first  sight, 
from  the  crystals  of  the  calcium  salt.  Such  instances  are  for- 
tunately very  rare.  Allowing  the  preparation  to  stand  a  few 
minutes  will  usually  permit  the  crystals  to  develop  and  their 
appearance  will  then  be  such  as  to  avoid  error.  If,  however,  the 
analyst  is  still  in  doubt  he  may  proceed  as  follows:  After  allowing 
sufficient  time  for  the  separation  of  almost  all  the  calcium  as 
CaS04  •  2  H20,  draw  off  the  supernatant  liquor,  add  to  the  residue 


290  ELEMENTARY   CHEMICAL   MICROSCOPY 

a  solution  of  ammonium  carbonate,  the  crystals  of  calcium 
sulphate  will  be  dissolved  and  highly  refractive  rhombs  and 
grains  of  calcium  carbonate  will  appear;  these  are  easily  found 
by  examining  the  preparation  between  crossed  nicols.  A  high 
power  is  generally  required. 

A  serious  interference  is  that  of  the  chlorides  of  the  trivalent 
metals.  In  the  presence  of  these  salts  in  large  amounts  it  is 
generally  advisable  to  proceed  thus:  Add  to  the  somewhat 
dilute  solution,  ammonium  acetate,  heat  to  boiling,  but  avoid 
long  or  violent  ebullition,  since  in  the  latter  case  the  precipitate 
formed  often  refuses  to  settle.  The  clear  liquid  is  then  sepa- 
rated from  the  precipitate  (by  drawing-off  on  the  slide,  filtration, 
or  by  means  of  the  centrifuge),  concentrated  if  necessary,  and 
tested  for  calcium  with  sulphuric  acid. 

Behrens  states  that  calcium  cannot  satisfactorily  be  detected 
in  the  presence  of  borates;  this  appears  to  be  true  when  only  a 
minute  quantity  of  calcium  is  present  with  a  high  percentage 
of  boron  and  other  salts;  in  such  an  event  test  by  Method  B. 

Strong  mineral  acids,  in  excess,  so  increase  the  solubility  of 
calcium  sulphate  as  to  require  evaporation  almost  to  complete 
dryness  before  the  crystals  of  tins  salt  appear.  The  addition 
of  a  fragment  or  two  of  sodium  acetate  or  of  ammonium  acetate 
is  always  necessary  in  such  cases  before  the  sulphuric  acid  drop 
is  allowed  to  flow  in.  This  method  of  mitigating  the  action  of 
the  free  acids,  also  reduces  the  delicacy  of  the  reaction  because 
of  the  formation  of  more  soluble  double  sulphates  of  calcium 
and  sodium  or  ammonium.  Hence  the  addition  of  an  excess  of 
a  soluble  sulphate  instead  of  sulphuric  acid  is  not  to  be  recom- 
mended. 

EXPERIMENTS. 

a.  Try  reaction,  in  the  manner  given  above,  on  salts  of  calcium  in  neutral 
solution. 

b.  Try  the  effect  of  precipitating  in  the  presence  of   free  HC1;    then   in  the 

presence  of  free  HN03. 

c.  Precipitate  with  dilute  H2S04,  then  heat,  adding  more  acid  if  necessary, 
until  white  fumes  are  given  off,  cool,  breathe  on  the  preparation  and  examine. 
Calcium  will  separate  either  as  the  salt  CaS04,  or  as  CaS04  •  H2S04.  The  crystal 
forms  most  frequently  met  with  are  thin,  rounded,  prism-like  plates  or  fusiform 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS        2QI 

crystals  with  tufted  ends.     This  modification  of  the  test  is  not  satisfactory  for 
Ca,  but  is  characteristic  for  Ba  and  for  Sr  (q.v.). 

d.  Try  testing  for  a  trace  of  Ca  in  the  presence  of  a  large  quantity  of  salts  of 
the  elements  of  Group  I. 

e.  Try  effect  of  a  solution  of  (NH4)2C03  on  crystals  of  CaS04  •  2  H20. 


B.   By  Means  of  Oxalic  Acid. 
Apply  the  reagent  according  to  Method  /,  page  251. 

The  oxalate  which  separates  at  room  temperature  from  neutral 
or  slightly  alkaline  solution  has  the  formula  CaC204  •  3  H20, 
and  belongs  to  the  tetragonal  system.  The  crystals  are  tiny, 
highly  refractive  octahedra,  or  rectangular  or  square  plates.  If 
rapidly  formed,  crosses  and  bundles  or  sheaves  of  crystallites 
will  be  seen.  From  hot  or  acid  solutions  a  monoclinic  oxalate 
CaC204  •  H20  separates  which  is  practically  valueless  as  an 
identity  test  for  calcium.  This  same  salt  appears  to  sometimes 
separate  if  a  large  excess  of  oxalic  acid  has  been  added.  In  addi- 
tion to  changing  the  crystal  form  free  mineral  acids  so  increase 
the  solubility  of  calcium  oxalate  as  to  sometimes  prevent  its 
precipitation. 

Strontium  gives  with  oxalic  acid  an  identical  reaction,  save  that 
the  crystals  of  strontium  oxalate  are  generally  somewhat  larger. 

Barium  oxalate  takes  the  form  of  fibrous  bundles  of  needles 
and  is  not  likely  to  be  mistaken  for  either  calcium  or  strontium. 

Zinc  under  certain  conditions  may  yield  a  zinc  oxalate  difficult 
to  distinguish  from  the  oxalates  of  calcium  and  strontium. 

Magnesium  oxalate  will  separate  in  forms  not  to  be  distin- 
guished from  calcium  oxalate  if  the  test  drop  contains  much 
acetic  acid,  but  in  the  absence  of  this  acid  magnesium  oxalate 
will  not  appear. 

Manganese  forms  groups  of  radiating  needles  (see  Manganese). 

Lead  oxalate  may  also  assume  forms  somewhat  resembling 
those  of  calcium  oxalate,  but  after  a  short  time  these  crystals 
grow  into  large,  well-developed  prisms. 

Silver  oxalate  separates  first  as  a  granular  deposit,  soon 
changing  to  crystals  of  a  great  variety  of  forms,  hexagonal 
plates,  six-sided  plate-like  prisms  and  stout  prisms  with  obliquely 
truncated  ends. 


292  ELEMENTARY   CHEMICAL  MICROSCOPY 

In  the  presence  of  stannic  chloride  Behrens  has  shown  that 
calcium  oxalate  assumes  the  form  of  tiny  oval  grains  exhibiting 
an  octahedral  tendency  while  strontium  yields  large  clear-cut 
beautifully  developed  tetragonal  octahedra  and  barium  gives 
short  stout  prisms  singly,  in  crosses  and  in  radiating  masses, 
or  if  much  barium  is  present,  fusiform  crystals  and  bundles  of 
radiating  needles  are  seen. 

Precautions . 

Oxalic  acid,  under  favorable  conditions,  can  cause  the  separa- 
tion of  oxalates  of  the  following  elements:  Gl,  Ca,  Sr,  Ba,  Mg, 
Zn,  Cd,  Tl;  rare  earths;  Sb,  Bi,  Sn,  Pb,  U,  Mn,  Fe,  Ni,  Co, 
Cu,  Ag. 

In  the  event  of  a  precipitate  of  doubtful  composition  being 
obtained,  draw  off  the  supernatant  liquid,  or  separate  by  means 
of  the  centrifuge,  and  add  to  the  residue  a  tiny  drop  of  dilute 
sulphuric  acid;  calcium  oxalate  is  dissolved  and  in  a  few  seconds 
the  characteristic  crystals  of  CaS04  •  2  H20  make  their  appear- 
ance. 

Owing  to  the  minute  size  of  the  crystals,  testing  for  calcium 
with  oxalic  acid  is  not  always  satisfactory.  As  an  offset  to  this 
disadvantage,  chlorides  of  the  trivalent  metals,  unless  in  con- 
centrated solution,  and  boric  acid  have  no  effect  other  than  a 
retardation  of  the  reaction.  A  small  amount  of  free  nitric  acid 
merely  greatly  retards  the  separation  of  the  oxalates  of  calcium 
and  strontium,  but  prevents  the  formation  of  barium  oxalate. 

EXPERIMENTS. 

a.  Try  reaction  after  the  manner  given  above,  on  a  salt  of  Ca  in  a  neutral 
solution.     Try  again  in  the  presence  of  free  HC1,  then  in  the  presence  of  free 

HNO3. 

b.  Precipitate  CaC204  •  3  H20,  draw  off  the  supernatant  liquor  and  treat  the 
residue  with  dilute  H2S04.  After  examining  the  preparation,  add  more  acid,  and 
heat  until  white  fumes  appear;  cool;  breathe  upon  the  preparation  and  examine 
again. 

STRONTIUM. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 
of  Strontium. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       293 

A.  ISOTROPIC.    Nitrate  (I). 

B.  ANISOTROPIC. 

Hexagonal. 

Tetragonal. 

Orthorhombic.  —  Chlorate;   sulphate. 

Monoclinic.  — Acetate;   chromate. 

Triclinic.  —  Chloride. 

DETECTION. 

A.   By  Means  of  Sulphuric  Acid. 

First  obtain  a  precipitate  of  strontium  sulphate  by 
Method  /,  page  251.  Examine  it  with  the  microscope  to  learn 
the  character  of  the  solid  phase.  Then  proceed  with  the  identi- 
fication of  the  practically  amorphous  precipitate  by  recrystal- 
lization  from  concentrated  sulphuric  acid  by  Method  XII, 
page  264,  or  from  concentrated  hydrochloric  acid  by  the  same 
method. 

Rarely,  strontium  sulphate  separates  in  the  cold  in  crystal 
form.  Heating  with  concentrated  sulphuric  acid  and  gently 
breathing  upon  the  preparation  yields  at  first  globular  masses 
and  tiny  rhombic  plates  of  a  salt  of  the  formula  SrS04  (or  some- 
times probably  SrS04  •  H2S04) .  These  tiny  plates  eventually 
develop  into  more  or  less  irregular  spindle-shaped  crystals, 
which  gradually  enlarge  at  the  middle  until  they  become  irregular 
crosses  with  two  very  short  arms.  The  appearance  is  very 
characteristic.  The  only  element  liable  to  lead  to  error  is  lead 
which  often  first  assumes  forms  closely  resembling  those  of  stron- 
tium, later  growing  into  crystallites  which  may  be  mistaken  for 
barium. 

Recrystallized  from  concentrated  hydrochloric  acid  strontium 
sulphate  has  an  entirely  different  habit.  Square  and  rectangular 
plates  appear  followed  by  thin  prisms  and  sheaves  of  slender 
pointed  crystals.  The  solubility  of  strontium  sulphate  in  hydro- 
chloric acid  is  quite  low,  hence  it  is  necessary  to  employ  a  large 
drop  of  the  solvent  and  therefore  it  is  seldom  that  all  the  pre- 
cipitate will  dissolve.  It  follows  that  to  obtain  the  best  results 
the  solvent  should  be  decanted  from  the  precipitate  immediately 


294  ELEMENTARY   CHEMICAL  MICROSCOPY 

after  heating,  and  before  crystallization  sets  in.  The  resulting 
crystals  are  quite  small  and  of  varied  form.  The  results  are 
less  satisfactory  than  with  sulphuric  acid,  but  there  is,  on  the 
other  hand,  the  advantage  that  barium  sulphate  is  practically 
insoluble  in  hydrochloric  acid.  It  is  of  course  essential  in  re- 
crystallizing  from  hydrochloric  acid  that  not  more  than  mere 
traces  of  free  sulphuric  acid  be  present.  Free  nitric  acid  should 
be  absent. 

Before  any  attempt  is  made  to  recrystallize  the  precipitate 
of  strontium  sulphate,  it  is  advisable,  and  usually  necessary, 
to  remove  any  calcium  which  may  be  present.  This  is  accom- 
plished by  extracting  the  precipitate  with  hot  water  in  which 
the  calcium  salt  is  soluble.  Unless  this  is  done,  peculiar  crystal 
forms  are  obtained  which  are  difficult  to  interpret. 

If  only  a  small  amount  of  barium  is  present,  characteristic 
crystals  of  strontium  sulphate  are  still  obtained  from  hot  sul- 
phuric acid,  but  much  barium  is  apt  to  alter  the  usual  crystal 
form,  although  the  appearance  of  the  crystals  separating  still 
suggests  the  strontium  sulphate  type.  An  excess  of  barium 
seems  to  cause  the  majority  of  the  crystals  to  assume  forms 
somewhat  resembling  barium  sulphate.  But,  in  general,  crystals 
of  both  strontium  and  barium  sulphate  can-  be  distinguished  in 
mixtures  of  these  two  elements. 

Any  lead  which  may  be  present  will  be  precipitated  in  an 
amorphous  condition  by  the  dilute  acid,  although  under  rare 
conditions  it  may  appear  crystalline.  Recrystallized  from  hot 
sulphuric  acid,  the  lead  sulphate,  as  stated  above,  will  separate 
in  forms  which  at  first  closely  resemble  those  of  strontium  sul- 
phate and  which,  later,  grow  to  forms  which  may  be  mistaken 
for  barium  sulphate.  Recrystallized  from  hydrochloric  acid 
there  is  less  danger  of  error.  If  in  doubt,  extract  the  precipi- 
tated sulphates  with  a  solution  of  potassium  or  sodium  hydrox- 
ide in  which  lead  sulphate  is  soluble. 

Silver  sulphate  will  appear  as  already  described  under  calcium. 
Hence  silver  as  well  as  most  of  the  lead  should  first  be  removed 
with  hydrochloric  acid. 

As  in  the  case  of  calcium,  chlorides  of  the  trivalent  metals 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       295 

and  salts  of  boric  acid  may  sometimes  interfere  with  the  forma- 
tion of  typical  crystals  of  strontium  sulphate. 

EXPERIMENTS. 

a.  To  a  drop  of  moderately  dilute  solution  of  SrCL.,  add  dilute  H2SO4  and 
examine  at  once. 

b.  Recrystallize  SrS04  from  H2S04  and  from  HC1. 

c.  Try  to  recrystallize  SrS04  from  HC1  in  the  presence  of  H2S04. 

d.  Make  a  mixture  of  Ca  and  Sr  salts  and  add  H2SO4.  Recrystallize  the 
product  from  H2S04  without  having  removed  the  Ca.  In  another  portion  remove 
the  Ca  by  extracting  with  boiling  water  and  then  recrystallize  the  residue. 


B.   By  Means  of  Oxalic  Acid. 

See  directions  given  under  calcium,  Method  B,  page  291. 
The  crystals  of  strontium  oxalate  are  similar  to  those  obtained 
with  calcium,  but  are  usually  distinctly  larger,  and  crosses, 
prisms,  and  four-pointed  rosettes  are  more  abundant  and  larger. 
The  crystals  are  either  tetragonal  or  monoclinic  depending  upon 
whether  formed  in  the  cold  or  separating  from  hot  solutions. 

Precautions. 

To  avoid  error  when  testing  with  oxalic  acid,  it  is  always  ad- 
visable, after  the  crystals  have  well  formed,  to  draw  off  the 
supernatant  solution  and  add  dilute  sulphuric  acid  to  the  pre- 
cipitate. If  no  crystals  of  calcium  sulphate  appear  after  a  few 
minutes,  add  more  acid  and  heat  until  white  fumes  appear,  care- 
fully observing  the  usual  precautions.  Transfer  the  drop  of  acid 
to  a  clean  slide,  breathe  on  the  drop  and  examine  for  fusiform 
crystals  of  strontium  sulphate. 

EXPERIMENTS. 

a.  Test  a  drop  of  SrCl2  solution  with  H2C204. 

b.  Treat  the  oxalate  thus  obtained  with  H2S04  and  recrystallize. 


BARIUM. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 
of  Barium. 

A.  ISOTROPIC.     Nitrate  (I). 

B.  ANISOTROPIC. 

Hexagonal.  —  Nitrite. 


296  ELEMENTARY   CHEMICAL   MICROSCOPY 

Tetragonal. 

Orthorhombic.  —  Chromate  (O  or  M). 

Monoclinic.  —  Chloride;   chlorate;  bromide;  ferro- 

cyanide;  acid-oxalate. 
Triclinic.  —  Acetate. 

DETECTION. 

A.   By  Means  of  Sulphuric  Acid. 

Read  fully  the  directions  and  comments  under  Calcium 
and  Strontium,  pages  289  and  290,  and  293  and  294. 

The  amorphous  or  semicrystalline  precipitate  first  obtained 
must  be  recrystallized  from  concentrated  sulphuric  acid  before 
identification  is  possible.  The  recrystallized  salt  appears  at 
first  as  tiny  rectangular  plates  and  X-like  crystallites.  In  this 
stage  of  development  it  may  be  mistaken  for  strontium  sulphate. 
Continue  breathing  upon  the  drop  of  acid;  under  the  influence 
of  the  moisture  absorbed  the  crystallites  grow  rapidly,  still 
retaining  their  X-like  shape  but  the  arms  of  the  X's  become 
feathered.  There  is  a  marked  tendency  for  two  adjacent  arms 
of  the  X  to  develop  much  more  rapidly  than  the  other  two. 
These  crystallites  grow  relatively  large  and  are  constant  and 
peculiar  to  barium. 

In  the  presence  of  certain  acids  or  acid  salts,  especially  from 
hot  solutions,  crystallites  of  barium  sulphate  may  sometimes  be 
obtained  immediately  upon  the  addition  of  dilute  sulphuric  acid. 

In  the  event  of  a  heavy  precipitate  being  obtained  with  the 
reagent,  it  is  wise  to  remove  a  small  portion  to  another  slide  for 
crystallization,  rather  than  attempt  to  dissolve  the  whole  mass. 

Recrystallization  in  the  presence  of  much  calcium  is  to  be 
avoided.     First  extract  the  calcium  sulphate  with  hot  water. 

In  the  presence  of  moderate  amounts  of  strontium  the  crys- 
tallites of  barium  sulphate  are  generally  not  well  formed.  If 
strontium  is  in  excess,  the  crystals  separating  from  the  hot  sul- 
phuric acid  have  the  general  type  of  strontium  sulphate,  but  are 
not  well  developed  and  exhibit  an  inclination  to  approach  the 
X-forms  of  barium  sulphate.  For  this  reason  it  is  advisable  to 
remove  any  strontium  which  may  be  present  by  repeatedly 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS        297 

heating  with  hydrochloric  acid,  in  which  strontium  sulphate  is 
soluble,  while  the  barium  compound  remains  undissolved  and 
can  then  be  recrystallized  by  heating  with  sulphuric  acid.  Even 
in  mixtures,  however,  it  is  almost  invariably  possible  to  find 
characteristic  forms  of  both  barium  and  strontium,  providing  the 
analyst  has  a  little  patience  and  carefully  examines  the  entire 
preparation. 

Any  lead  sulphate  which  may  be  present  will  appear,  first,  in 
crystals  very  suggestive  of  strontium  sulphate,  then,  in  a  short 
time,  in  larger  crystallites  which  may  at  times  be  mistaken  for 
barium  sulphate.  Treatment  with  hydrochloric  acid,  or,  better, 
with  sodium  hydroxide,  will  remove  the  lead,  leaving  the  barium 
salt  unacted  upon. 

Precautions. 

It  is  sometimes  desirable  to  apply  other  tests  to  the  precipitated 
sulphate  in  order  to  confirm  the  presence  of  barium.  In  such  an 
event,  transfer  the  washed  precipitate  to  platinum  foil  or  to  a 
platinum  cup  and  fuse  with  potassium  carbonate.  The  fused 
mass  is  then  extracted  with  water  and  the  residue  of  barium 
carbonate  dissolved  in  hydrochloric  acid.  This  solution  can 
then  be  tested  for  barium  by  any  of  the  tests  given  below. 

Since  chlorides  of  the  trivalent  metals  sometimes  interfere 
with  the  formation  of  characteristic  crystals  of  barium  sulphate, 
it  is  advisable  to  decant  the  supernatant  liquor  after  the  addi- 
tion of  the  reagent  and  before  heating  with  an  excess  of  the  acid. 
When  dealing  with  unknown  mixtures  it  is  always  best  to  pro- 
ceed in  this  manner. 

EXPERIMENTS. 

c.   Try  above  method  on  a  simple  salt  of  Ba. 

b.  Make  a  mixture  of  salts  of  Ca  and  Ba,  recrystallize  at  once  without  remov- 
ing the  Ca.  From  another  portion  remove  the  Ca  with  hot  water  and  recrystal- 
lize the  residue. 

c.  Try  a  mixture  of  Sr  and  Ba.  Remove  the  Sr  by  treating  with  HC1  and  re- 
crystallize  the  residue. 

d.  Try  a  mixture  of  Ca,  Sr  and  Ba,  recrystallizing  at  once,  then  removing  in 
turn  the  Ca  with  hot  water  and  the  Sr  with  HO. 

e.  After  having  tried  the  other  reactions  for  Ba  fuse  some  BaS04  with  K2COs 
and  proceed  as  directed  above. 


298  ELEMENTARY   CHEMICAL   MICROSCOPY 

B.   By  Means  of  Oxalic  Acid. 

Read  carefully  the  discussion  of  this  test  as  given  under 
Calcium  and  Strontium,  pages  292  and  293. 

Barium  oxalate  BaC204  •  «H20  forms  large  branching  aggre- 
gates, radiating  bundles  of  branching  crystallites  and  sheaves  of 
bristling  fibrous  needles.  Rarely,  well-developed  monoclinic 
prisms  may  be  obtained.  The  branching  crystallites  are  char- 
acteristic of  barium  and  are  never  given  by  calcium  or  by 
strontium. 

Precautions. 

The  solution  to  be  tested  should  be  neutral;  even  a  very  little 
trace  of  acid  is  apt  to  prevent  the  separation  of  the  character- 
istic crystals. 

If  no  crystals  appear  after  a  short  time,  add  a  fragment  of 
sodium  or  ammonium  acetate. 

When  calcium  or  strontium  are  present  the  characteristic 
crystal  forms  of  barium  oxalate  will  rarely  be  obtained.  Re- 
course may  then  be  had  to  testing  in  dilute  nitric  acid.  From 
nitric  acid  solutions  the  barium  salt  will  not  separate,  while  the 
oxalates  of  calcium  and  strontium  will  slowly  crystallize  in  their 
usual  form.  After  allowing  sufficient  time  for  the  complete 
separation  of  calcium  and  strontium,  decant,  concentrate  the 
solution  and  add  sodium  acetate.  Barium  oxalate  now  appears, 
usually  in  the  form  of  rosettes  of  thin  prisms. 

Barium  oxalate,  like  the  oxalates  of  calcium  and  strontium, 
assumes  different  crystal  forms,  according  as  the  test  drop  is  hot 
or  cold.  Hot  solutions  give  rise  to  the  production  of  strongly 
polarizing  orthorhombic  plates. 

Since,  in  order  to  facilitate  the  separation  of  barium  oxalate, 
sodium  acetate  has  been  added,  it  is  well  to  bear  in  mind  that 
there  is  danger  of  interference  from  members  of  the  magnesium 
group. 

Borates  present  in  the  test  drop,  if  in  large  amount,  may 
prevent  the  formation  of  characteristic  crystals  of  barium 
oxalate. 

Although  chlorides  of  iron  and  aluminum  have,  as  has  been 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS        299 

stated,  no  deleterious  influence  on  the  precipitation  of  the  oxa- 
lates of  calcium  and  strontium,  we  meet,  in  the  case  of  barium, 
with  a  most  interesting  and  remarkable  reaction.  Owing  to  the 
formation  of  double  oxalates  of  barium  and  iron  or  barium  and 
aluminum,  instead  of  the  typical  fibrous  bundles  of  needles 
and  crystallites,  there  are  now  obtained  tufts  and  bunches  of 
very  long  exceedingly  fine  curving  hair-like  crystals  (trichites) 
of  characteristic  appearance.  The  chemical  composition  and 
formulas  of  these  compounds  have  not  yet  been  definitely  ascer- 
tained. 

In  order  to  obtain  this  interesting  compound,  proceed  as 
follows:  To  the  test  drop  containing  barium,  add  ferric  chloride 
in  sufficient  amount  to  impart  a  faint  but  distinctly  yellow  color; 
then  add  a  fragment  or  two  of  sodium  or  ammonium  acetate; 
stir.  The  yellow  color  should  now  have  changed  to  a  reddish 
tint.  Into  this  drop,  thus  prepared,  cause  a  drop  of  oxalic  acid 
to  flow.  Tufts  and  sheaves  of  very  fine  hairs  soon  appear.  The 
hairs  rapidly  grow  longer  and  longer  and  soon  begin  to  curve  in 
a  most  peculiar  manner.  The  presence  of  calcium  or  strontium, 
or  both,  in  even  large  amounts  does  not  appear  to  have  any 
serious  influence  on  the  formation  of  this  double  oxalate  of 
barium  and  iron,  save  that  its  separation  is  often  somewhat  re- 
tarded. In  such  mixtures  the  oxalates  of  calcium  and  strontium 
first  appear  in  their  usual  form,  then  after  a  time  the  hair-like 
tufts  of  the  double  oxalate  appear.  If  the  quantity  of  barium 
is  quite  small,  in  proportion  to  the  iron,  little  rosettes  of  radiating 
needles  are  obtained,  separating  near  the  edges  of  the  drop. 

Aluminum  gives  rise  to  the  formation  of  a  similar  product, 
but  the  crystal  masses  are  colorless,  while  those  of  the  iron  salt 
are  light  brown. 

EXPERIMENTS. 

a.  Test  a  salt  of  Ba  with  H2C2O4,  in  both  hot  and  cold  solutions. 

b.  Make  a  mixture  of  Ca,  Sr,  Ba.  Add  H2C204.  Repeat  the  experiment  in 
HNO3  solution;  after  a  few  minutes,  decant  the  clear  solution,  concentrate  slightly 
and  add  NaC2H302. 

c.  Try  the  effect  of  the  presence  of  FeCl3  on  the  precipitation  of  oxalates  of  Ca, 
Sr,  Ba;  first  each  element  separately,  then  in  mixtures  of  Ca  and  Ba;  Sr  and  Ba; 
Ca,  Sr  and  Ba. 


300  ELEMENTARY   CHEMICAL   MICROSCOPY 

d.  If  barium  borate  is  at  hand,  try  testing  it  for  Ba. 

e.  Try  H2C2O4  on  a  salt  of  Mg,  then  add  an  excess  of  HC2H302  to  the  test  drop 
and  examine  again. 

/.   Test  salts  of  Zn,  Cd,  Pb  and  Ag. 


BEHAVIOR  OF  CALCIUM,  STRONTIUM  AND  BARIUM 
TO  OTHER  IMPORTANT  REAGENTS. 

The  tests  already  given  are  generally  ample  for  the  proper 
identification  of  the  alkaline  earths,  but  occasionally  problems 
arise  where  supplementary  or  alternate  methods  are  desirable. 
The  following  reactions  have,  therefore,  been  included  both  on 
account  of  their  applicability  to  the  examination  of  unknown 
material  and  because  of  the  further  light  they  throw  upon  the 
similarities  and  differences  between  the  members  of  the  Calcium 
Group. 

Behavior  with  Potassium  Ferrocyanide. 

The  reagent  is  applied  by  Method  /,  page  251,  to  the  test 
drop  acidulated  with  acetic  acid  and  containing  a  little  ammo- 
nium chloride. 

Calcium  yields  tiny  rectangular  or  square  plates. 

Strontium  fails  to  form  a  ferrocyanide  under  the  conditions 
given  above. 

Barium  yields  large,  clear,  transparent,  yellow  rhombs  prob- 
ably belonging  either  to  the  orthorhombic  or  to  the  triclinic 
system,  depending  upon  the  amount  of  water  of  hydration. 

The  salts  separating  are  double  ferrocyanides  to  which  the 
following  formulas  have  been  ascribed:  K2CaFe(CN)6  •  3  H20 
and  K2BaFe(CN)6  •  5  H20  (0?)  or  K2BaFe(CN)6  •  3  H20  (Tr). 
As  usually  obtained  the  barium  salt  extinguishes  parallel  to  a 
line  drawn  through  the  acute  angles  of  the  rhombs.  This  fact 
enables  the  analyst  to  readily  differentiate  between  the  double 
barium  salt  and  chance  separation  of  the  reagent  (M). 

Free  mineral  acids  must  be  absent. 

Potassium  ferrocyanide,  though  giving  a  neat  reaction  with 
pure  salts  of  barium,  is  of  little  value  when  dealing  with  mix- 
tures.    It  is  then  often  difficult  to  avoid  the  precipitation  of 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS        30 r 

calcium  with  the  barium,  particularly  if  much  ammonium  chlor- 
ide is  present,  or  if  much  sodium  acetate  has  been  added  to 
mitigate  the  action  of  mineral  acids. 

From  mixtures,  strontium  may  sometimes  be  precipitated  in 
an  amorphous  condition  if  the  solution  is  quite  concentrated, 
and  may  thus  interfere  with  the  test.  Pure  salts  of  strontium 
give,  even  in  very  concentrated  solutions,  only  a  granular  deposit 
consisting  of  globular  masses,  exhibiting  no  distinguishable 
crystal  form. 

Magnesium  is  precipitated  from  ammoniacal  solutions,  but 
neither  from  acid  nor  from  neutral  solutions;  hence  the  pres- 
ence of  this  element  will  not  mask  the  test  for  barium. 

In  addition  to  calcium  and  strontium,  there  are  a  number  of 
other  elements,  which,  if  present,  will  either  be  precipitated  in 
insoluble  form  or  will  interfere  with  the  formation  of  the  barium 
crystals.  In  this  list  the  most  frequently  met  with  will  be  lead, 
iron,  zinc,  rare  earths  and  less  often  copper,  mercury,  uranium, 
and  titanium. 

EXPERIMENTS. 

a.  Crystallize  a  little  of  the  reagent  K4Fe(CN)6,  alone,  and  determine  its  optical 
properties. 

b.  Try  reagent  on  pure  salts  of  Ca,  Sr,  Ba,  using  both  dilute  and  concentrated 
solutions.  Try  again,  this  time  proceeding  as  directed  above,  using  HC2H302  and 
NH4CI. 

c.  Try  the  reagent  on  mixtures  of  Ca  and  Sr,  Ca  and  Ba,  Sr  and  Ba. 

d.  Try  effect  of  the  reagent  on  salts  of  Pb,  Zn  and  Fe.  Then  make  mixtures  of 
Ba  and  these  elements  and  test. 

e.  Make  a  preparation  of  K2BaFe(CN)6,  measure  the  angles  of  the  crystals  and 
determine  the  optical  properties  of  the  compound. 

Behavior  with  Ammonium  or  Potassium  Bichromate. 

The  reagent  is  applied  to  the  test  drop  in  solid  form,  Method 
III,  page  252. 

From  acetic  acid  solution,  barium  chromate  BaCr04  is  im- 
mediately precipitated,  orthorhombic,  in  the  form  of  minute 
light-yellow  globular  masses,  or  tiny  rods  with  rounded  ends. 
Strontium  chromate  will  not  separate  from  acid  solutions  but 
only  from  -neutral  or  slightly  alkaline  solutions.     Calcium  is 


302  ELEMENTARY   CHEMICAL   MICROSCOPY 

precipitated  by  bichromate  from  neither  acid,  neutral  nor  am- 
moniacal  solutions. 

The  strontium  salt  of  the  formula  SrCr04  appears  from 
ammoniacal  solution  as  exceedingly  tiny  yellow  globulites  or 
dumb-bell-like  aggregates;  it  is  dimorphic,  being  either  ortho- 
rhombic  or  monoclinic.  If  the  former,  it  is  isomorphous  with 
the  barium  salt. 

When  this  test  is  used,  acidify  the  dilute  drop  with  acetic 
acid,  then  add  the  fragment  of  bichromate.  Do  not  stir,  and 
avoid  rubbing  the  glass  with  rod  or  wire.  Barium  chromate 
separates  at  once  if  present.  After  several  minutes  decant  if 
a  precipitate  has  formed.  To  the  decanted  solution  or  clear 
drop  add  a  small  drop  of  ammonium  hydroxide  and  examine 
the  preparation  for  dumb-bells  of  strontium  chromate. 

If  both  barium  and  strontium  are  believed  to  be  present  it 
is  best  to  warm  the  preparation  to  cause  as  complete  a  precipi- 
tation of  barium  chromate  as  possible  before  adding  the  am- 
monium hydroxide,  but  care  must  be  taken  to  avoid  unduly 
concentrating  the  drop.  It  is  also  usually  better  to  allow  the 
ammonium  hydroxide  to  flow  into  the  drop  from  one  side  rather 
than  add  it  directly  to  the  middle  of  the  drop. 

Normal  potassium  chromate  produces,  with  barium  salts,  a 
precipitate  similar  to  that  obtained  with  dichromate,  but  is  not 
to  be  recommended  as  a  reagent  because  of  its  property  of  also 
precipitating  strontium  compounds  in  acid  solution. 

Ordinarily  the  precipitate  of  barium  chromate  is  mostly 
amorphous  in  appearance.  Here  and  there,  however,  will  be 
found  areas  where  there  are  recognizable  crystals.  A  high 
power  is  always  required  for  the  recognition  of  the  form  of  the 
crystals,  hence  the  drop  to  be  studied  must  be  spread  out  quite 
thin. 

Free  mineral  acids  interfere  with  the  test. 

In  addition  to  barium  and  strontium,  it  must  be  remembered 
that  dichromate  will  also  yield  crystalline  precipitates  with 
silver,  lead,  mercury  and  thallium,  but  in  these  cases  nitric  acid 
may  be  present. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       303 

EXPERIMENTS. 

a.  Try  reaction  on  salts  of  Ba,  Sr  and  Ca,  in  acid,  neutral  and  ammoniacal  solu- 
tions, and  both  in  concentrated  and  in  dilute  solutions. 

b.  Try  mixtures  of  Ca  and  Ba,  Sr  and  Ba;  use  solutions  acidified  with  HC2H302, 
decant  the  clear  solution,  and  to  it  add  NH4OH. 

c.  Try  the  reagent  upon  Ba  and  Sr  salts  in  HN03  solution.  Then  try  it  upon 
Ag,  Pb  and  mercurous  salts  in  HN03  solution. 

Behavior  with  Primary  Sodium  Carbonate. 

An  almost  saturated  solution  of  the  reagent  is  added  to  the 
dilute  ammoniacal  test  drop  by  Method  I,  page  251. 

Calcium  carbonate  CaC03  separates  in  very  small  disks  and 
rhombs  (H  or  O). 

Strontium  yields  spherulites  often  of  considerable  size. 

Barium  separates  as  minute  spider-like  aggregates  and  tiny 
spherulites,  the  latter  often  uniting  to  form  spindles  and  dumb- 
bell-like masses. 

The  addition  of  the  reagent  in  solid  form  gives  nearly  as  good 
results. 

Warming  the  preparation  increases  the  rapidity  of  the  reac- 
tion and  leads  to  the  formation  of  better  crystals. 

Unless  the  test  drop  is  quite  dilute  an  amorphous  precipitate 
results. 

Ammonium  carbonate  can  be  substituted  for  the  sodium  salt; 
the  crystals  then  differ  but  little  if  any  from  those  obtained  as 
above,  but  normal  sodium  carbonate  gives  amorphous  precipi- 
tates only  and  therefore  should  never  be  employed. 

When  simple  salts  of  the  elements  calcium,  strontium  and  ba- 
rium are  employed  it  is  not  at  all  difficult  to  distinguish  between 
them  by  testing  with  primary  sodium  carbonate  (or  ammonium 
carbonate).  But  if  two  or  more  of  these  elements  are  present 
the  method  fails,  characteristic  crystals  being  the  exception. 

In  the  presence  of  a  great  excess  of  the  reagent  a  double 
carbonate  of  calcium  and  sodium  separates,  having  the  formula 
CaC03  •  Na2C03  •  5  H20,  which  crystallizes  in  stout  monoclinic 
prisms  somewhat  resembling  the  short,  thin  prisms  of  calcium 
sulphate.  Strontium  and  barium  prevent  the  formation  of  the 
double  salt. 


304  ELEMENTARY   CHEMICAL  MICROSCOPY 

Elements  of  the  magnesium  group  interfere.  Lithium  like- 
wise interferes.  But  the  chlorides  of  iron  and  aluminum  and 
the  salts  of  boric  acid  have  no  appreciable  effect  on  the  reaction. 

When  in  doubt  as  to  the  nature  of  a  precipitate  formed  by  the 
treatment  with  HNaC03,  decant  the  supernatant  solution,  which 
is  easily  done  since  the  crystals  of  calcium  carbonate  adhere  to 
the  glass  slide,  wash  the  residue,  and  then  add  dilute  sulphuric 
acid.  If  the  precipitate  is  due  to  calcium,  characteristic  crys- 
tals of  CaS04  •  2  H20  appear. 

Primary  sodium  carbonate  is  of  more  value  as  a  group  reagent 
than  as  an  identification  test.  Moreover,  chance  formations  of 
crystals  of  alkali  carbonates  may  be  met  with  in  the  progress 
of  the  systematic  analysis  of  unknown  material,  particularly 
when  testing  for  zinc  (q.v.). 

MAGNESIUM. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 
of  Magnesium. 

A.  ISOTROPIC. 

B.  ANISOTROPIC. 

Hexagonal.  —  Pyroantimonate. 

Tetragonal.  —  Fluoride. 

Orthorhombic.  — Ammonium  -magnesium  phosphate; 

sulphate;   primary  tartrate. 
Monoclinic. — Acetate;  chloride;  nitrate;  primary 
phosphate ;      ammonium-magnesium     sul- 
phate;     potassium-magnesium     sulphate; 
normal  tartrate. 
Triclinic. 

DETECTION. 
A.   By  Means  of  Uranyl  Acetate  and  Sodium  Acetate. 

This  test  has  already  been  described  at  length  under 
Sodium,  Method  A,  page  275. 


B.   By  Means  of  Secondary  Sodium  Phosphate  {HNa»_PO^) 
in  Ammoniacal  Solution. 
For  the  reaction  see  Ammonium,  page  286. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       305 

The  detection  of  magnesium  in  simple  salts  is  comparatively 
easy  and  rapid,  since  characteristic  crystals  are  readily  obtained, 
but  its  microchemical  identification  in  complex  mixtures  is  usu- 
ally a  matter  of  not  a  little  difficulty,  in  as  much  as  this  element 
is  commonly  associated  with  others,  closely  related,  which  are 
prone  to  interfere  with  or  prevent  the  formation  of  typical 
crystals  with  the  reagents  employed  for  its  recognition. 

Two  methods  are  available,  the  choice  of  procedure  depending 
upon  the  nature  of  the  salts  present  in  the  drop  to  be  tested. 
In  all  cases  where  there  is  a  doubt  as  to  the  probable  composition 
of  the  material  to  be  examined,  it  is  best  to  have  recourse  at 
once  to  the  modification  II.1 

I.  To  the  solution  of  the  material  to  be  tested,  which  must 
not  be  too  concentrated,  add  several  fragments  of  ammonium 
chloride;  stir;  then  add  a  very  slight  excess  of  ammonium  hy- 
droxide, and  warm  the  preparation.  (If  a  precipitate  results  it 
is  best  to  draw  off  the  clear  solution.)  To  the  warm  solution  add  a 
small  crystal  of  secondary  sodium  phosphate.  Crystals  of  am- 
monium magnesium  phosphate  NH4MgP04  •  6  H20  soon  appear. 

II.  To  the  solution  to  be  tested  add  a  fragment  or  two  of 
citric  acid,  stir  until  dissolved,  then  add  an  excess  of  ammonium 
hydroxide.  Evaporate  to  dryness.  To  the  residue  add  dilute 
ammonium  hydroxide.  Warm;  then  add  a  very  small  frag- 
ment of  secondary  sodium  phosphate.  Crystals  of  ammonium 
magnesium  phosphate  separate. 

The  crystals  of  the  ammonium  magnesium  phosphate  sepa- 
rate as  skeletons  and  hemimorphic  forms  of  the  orthorhombic 
system  (see  Ammonium). 

It  should  be  remembered  that  a  number  of  elements  are 
precipitated  by  phosphates  in  alkaline  solution;  the  most  fre- 
quently met  with  in  the  course  of  microchemical  analyses,  either 
in  the  substance  to  be  tested,  or  present  as  reagents  from  previous 
tests,  are,  doubtless,  lithium,  members  of  the  calcium  and  mag- 
nesium groups,  trivalent  metals,  manganese,  nickel,  cobalt,  tin, 
lead,  silver,  copper,  and  uranium.2     Of  these  elements,  lithium, 

1  Romijn,  Zeit.  anal.  Chem.,  37,  300. 

2  Most  of  these  elements  will  generally  have  been  removed  in  the  progress  of 
the  analysis  before  the  addition  of  the  sodium  phosphate. 


306  ELEMENTARY   CHEMICAL   MICROSCOPY 

iron,  manganese,  cobalt  and  nickel  form,  with  ammonium  and 
phosphoric  acid,  salts  of  similar  composition  to,  and  isomorphous 
with,  the  magnesium  salt. 

The  ammonium  glucinum  phosphate,  ammonium  zinc  phos- 
phate and  ammonium  cadmium  phosphate  are  not  precipitated 
in  crystal  form. 

The  advantage  of  employing  modification  II  lies  in  the  fact 
that  owing  to  the  presence  of  ammonium  citrate,  there  is  little 
danger  of  the  interference  of  the  elements  listed  above.  If  in 
following  this  method,  the  residue  after  evaporation  is  not  com- 
pletely soluble  in  the  ammonium  hydroxide  solution,  it  is  best, 
though  not  essential,  to  decant  the  clear  liquid  before  adding 
to  it  the  sodium  phosphate. 

Reactions  I  and  II  work  equally  well  in  the  cold,  but  are  then 
a  trifle  slower.  Generally,  an  amorphous  precipitate  is  at  first 
produced  which  begins  to  crystallize  in  a  few  seconds.  The 
formation  of  merely  an  amorphous  precipitate  must  never  be 
taken  as  evidence  of  the  presence  of  magnesium. 

In  the  presence  of  phosphates  the  detection  of  magnesium 
becomes  quite  difficult,  particularly  if  other  elements  are  present 
which  form  phosphates  insoluble  in  ammonium  hydroxide.  If 
arsenates  are  also  present,  a  still  further  complication  arises,  for, 
as  we  have  already  seen,  double  ammonium  arsenates  of  calcium, 
zinc,  etc.,  are  formed,  which  are  isomorphous  with  ammonium 
magnesium  phosphate. 

Of  course  it  may  happen  that  in  some  cases  the  mere  addition 
of  ammonium  hydroxide  will  cause  the  separation  of  character- 
istic crystals  of  ammonium  magnesium  phosphate.  Generally, 
however,  it  is  first  necessary  to  remove  the  phosphoric  acid. 
This  can  be  accomplished  by  tin  and  nitric  acid,  or  by  means  of 
ammonium  tungstate  and  nitric  acid  (see  Phosphates,  page  38c). 

Precautions. 

In  I,  the  reaction  sometimes  fails  for  lack  of  sufficient  ammo- 
nium chloride,  magnesium  hydroxide  being  precipitated.  A 
slight  excess  of  this  salt  will  do  no  harm. 

Both  modifications  fail  if  there  is  an  insufficiency  of  ammo- 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       307 

nium  hydroxide,  for  it  should  be  remembered  that  there  must 
be  not  only  enough  ammonium  present  to  unite  to  form  the 
proper  compound,  but  that  this  latter  salt  will  not  separate  save 
in  alkaline  solution. 

It  must  also  be  borne  in  mind  that  the  use  of  too  strong 
ammonium  hydroxide  in  excess  so  reduces  the  solubility  of 
many  salts  as  to  cause  their  separation.  Hence  it  is  necessary 
to  avoid,  in  reactions  of  this  character,  deciding  too  hastily  as 
to  the  result  of  a  test. 

EXPERIMENTS. 

a.  Try  modification  I  on  a  solution  of  MgS04,  then  try  it  on  salts  of  Fe,  Mn, 
Co,  Ni,  Al,  Zn  and  Cd.  Repeat  the  experiments,  this  time  adding  the  HNa2P04 
before  the  NH4OH. 

b.  Try  modification  II  upon  the  same  salts  and  combinations  used  in  a. 

c.  Make  mixtures,  trying  various  combinations  of  the  above  with  members  of 
Groups  1  and  II. 


ZINC. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 
of  Zinc. 

A.  ISOTROPIC. 

B.  ANISOTROPIC. 

Hexagonal. 

Tetragonal. 

Orthorhombic.  —  Chromate;  sulphate.1 

Monoclinic.  —  Acetate;  potassium-zinc  sulphate. 

Triclinic. 

DETECTION. 

A.   By  Means  of  Ammonium  Mercuric  Sulphocyanate. 
Apply  the  reagent  by  Method  /,  page  251. 
This  reagent  furnishes  us  with  one  of  the  best  and  the  most 
generally  useful  methods   for  detecting   the  presence   of   zinc, 
copper,  cadmium  and  cobalt,  and  will  also  furnish  evidence  of 
the  presence  of  iron,  silver,  lead  and  gold. 

1  If  formed  in  the  presence  of  ferrous  sulpnate,  monociinic. 


3o8  ELEMENTARY  CHEMICAL  MICROSCOPY 

For  the  qualitative  examination  of  simple  salts  and  alloys  it 
leaves  little  to  be  desired,  but  in  the  analysis  of  minerals,  it  is 
better  to  employ  the  carbonate  test  first,  then  corroborate  with 
the  sulphocyanate  reagent. 

Upon  adding  a  rather  concentrated  solution  of  the  reagent  to 
a  dilute  solution  of  the  metals  listed  above  the  following  results 
are  obtained: 

Zinc  yields  an  almost  instantaneous  precipitation  of  the  com- 
pound Zn(CNS)2  •  Hg(CNS)21  in  pure  white  feathery  crosses  and 
branching  feathery  aggregates.  These  skeleton  crystals,  when 
thick,  appear  black  by  transmitted  light  and  snow  white  by 
reflected  light.  The  normal  crystal  of  the  double  sulphocyanate 
of  zinc  and  mercury  is  said  to  be  a  right-angled  prism  of  the 
orthorhombic  system,  but  under  the  conditions  which  obtain  in 
ordinary  practice,  only  skeleton  and  dendritic  forms  will  be 
seen. 

Neither  magnesium  nor  aluminum  interfere  with  this  test, 
save  that  when  magnesium  is  present  in  very  large  amount,  the 
separation  of  the  zinc  salt  is  retarded,  and  that  aluminum  under 
similar  conditions  renders  the  skeleton  crystals  of  the  zinc  salt 
somewhat  less  feathery. 

When  zinc  alone  is  present  the  crystals,  as  has  been  stated 
above,  are  snow  white  and  of  the  form  described;  but  if  copper 
is  present  in  minute  amount,  the  crystals  of  the  zinc  salt  are 
colored  chocolate  brown  without  undergoing  any  change  of 
form.  These  brown  crystals  begin  to  appear  after  the  white 
ones  have  separated.  More  copper  than  sufficient  to  yield  the 
brown  tint  produces  black  crystals  of  modified  form;  still  a 
greater  proportion  of  copper  completely  changes  the  appearance 
of  the  crystals,  and  jet  black  spheres  and  botryoidal  masses 
result.  Finally  a  point  is  reached  where  crystals  of  copper 
mercuric  sulphocyanate  predominate,  accompanied  by  the  black 
crystals  just  mentioned.  In  all  cases,  however,  because  of  the 
much  lower  solubility  of  the  zinc  compound  than  that  of  the 

1  This  salt  is  generally  given  as  anhydrous.  Recent  work  seems  to  throw  some 
doubt  upon  this  and  to  indicate  the  presence  of  one  molecule  or  less  of  water  of 
hydration. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS        309 

other  complex  salts  formed,  there  will  always  be  formed  some  of 
the  typical  uncolored  zinc  mercury  sulphocyanate. 

Copper  alone  yields  beautiful  branching  dendrites  and  radiat- 
ing masses  of  acicular  prisms,  yellowish  green  in  color.  The 
reaction  is  sensitive  and  beautiful  and  constitutes  one  of  the 
most  satisfactory  tests  available  for  the  identification  of  copper. 

The  change  in  color  due  to  the  solid  solution  of  the  copper 
salt  Cu(CNS)2  •  Hg(CNS)2  •  H20,  in  the  zinc  salt  is  a  most  inter- 
esting one  and  one  for  which  no  really  satisfactory  explanation 
is  yet  at  hand. 

The  cobalt  salt  enters  into  the  zinc  salt  in  solid  solution  to 
yield  light  blue  crystals.  With  very  small  amounts  the  color 
is  exceedingly  faint  and  the  crystal  form  unchanged,  but  as  the 
proportion  of  cobalt  increases,  the  skeleton  crystals  of  the  zinc 
salt  become  deeper  and  deeper  blue,  simpler,  less  feathery,  and 
gradually  assume  the  color  and  appearance  of  the  normal  cobalt 
mercuric  sulphocyanate.  As  in  the  case  of  the  copper-zinc 
compound,  these  blue  crystals  are  doubtless  cases  of  solid  solu- 
tion, but  the  theory  of  isomorphous  mixture  is  more  tenable  in 
this  case  than  in  that  where  copper  is  present. 

Cobalt  alone  yields  deep  blue-black  orthorhombic  prisms, 
Co(CNS)2  •  Hg(CNS)2,  usually  imperfectly  developed  and  unit- 
ing to  form  star-like  clumps  and  radiating  masses.  This  con- 
stitutes a  valuable  method  for  differentiating  cobalt  from  nickel, 
since  nickel  yields  no  double  sulphocyanate  crystals  under  the 
conditions  which  obtain  in  microchemical  testing. 

Small  amounts  of  zinc  in  the  presence  of  much  cobalt  cannot 
be  detected  by  this  reagent. 

Cadmium  yields  Cd(CNS)2  •  Hg(CNS)2  in  brilliant  colorless, 
probably  orthorhombic  prisms,  usually  several  times  as  long 
as  broad  but  the  appearance  of  these  prisms  varies  with  the 
conditions  which  obtain  at  the  time  of  their  formation,  as,  for 
example,  the  concentration,  depth  of  the  test  drop,  amount  of 
reagent  added,  acidity,  etc.  These  variations  are,  however, 
not  of  a  kind  to  render  the  test  doubtful,  long  prisms,  either 
singly  or  in  groups  being  the  rule. 

Even  a  small  amount  of  cadmium  destroys  the  feathery  and 


310  ELEMENTARY   CHEMICAL  MICROSCOPY 

branched  character  of  the  skeletons  of  the  zinc-mercury  sulpho- 
cyanate,  owing  to  the  formation  of  mixed  crystals,  and  there 
generally  results  crystallites  of  the  shape  of  an  arrowhead. 
Small  amounts  of  zinc  in  the  presence  of  much  cadmium  will 
usually  escape  detection. 

Much  nickel  modifies  the  crystals  of  the  double  zinc  salt  in 
the  same  manner  as  cadmium.  With  much  nickel  and  very 
little  zinc  only  spherulites  are  obtained. 

The  presence  of  both  copper  and  cobalt  in  a  solution  contain- 
ing zinc  gives  rise  to  the  formation  of  mixed  crystals  of  very 
peculiar  color  and  form.  These  peculiarities  are  accentuated 
when  cadmium  is  also  present.  The  experienced  worker  thus 
will  have  little  difficulty  in  detecting  a  number  of  elements  in  one 
single  operation. 

Manganous  salts  in  excessively  concentrated  solutions  con- 
taining a  trace  of  free  sulphuric  acid  yield  crystals  closely  re- 
sembling those  of  the  cadmium  double  salt. 

Ferrous  compounds,  if  only  in  very  small  amount,  do  not 
interfere  with  the  formation  of  the  typical  crystals  of  the  zinc 
salt  but  in  high  per  cent  there  will  usually  be  obtained  radiating 
groups  or  feathery  dendrites  closely  resembling  the  copper  salt. 

Ferric  salts  always  yield  a  pink  or  red  color  and  have  no  effect 
upon  the  zinc  compound  until  a  concentration  is  reached  such 
that  a  deep  blood  red  color  appears.  Under  such  conditions  the 
zinc-mercury  sulphocyanate  first  separates  as  a  deep  reddish 
brown  salt,  jet  black  by  transmitted  light,  yet  still  retaining  the 
typical  feathery  drendritic  form,  but  in  a  few  seconds  these 
undergo  a  sudden  and  remarkable  change  into  masses  of  curving 
branching  filiform  crystals.  This  is  especially  marked  in  test 
drops  containing  sodium  or  ammonium  acetate. 

Lead,  unless  present  in  large  amount,  usually  seems  to  have 
little  or  no  effect  on  the  zinc  reaction.  Under  some  conditions 
it  seems  to  interfere,  however,  and  it  is,  therefore,  always  best 
to  first  remove  the  lead  by  means  of  dilute  sulphuric  acid.  Add 
the  acid,  decant  or  filter;  evaporate  the  clear  solution  to  dry- 
ness; fume  off  the  free  sulphuric  acid;  dissolve  in  water;  add 
ammonium  acetate,  and  test  as  above. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       311 

Silver  gives  with  the  reagent  a  white  amorphous  precipitate, 
soon  crystallizing  in  the  form  of  small,  thin,  slender  prisms 
with  square  or  oblique  ends,  somewhat  resembling  those  of  the 
cadmium-mercury  salt,  but  very  much  smaller  than  the  latter. 
In  the  presence  of  silver  the  test  for  zinc  is  sometimes  masked. 
In  such  an  event,  first  remove  the  silver  with  hydrochloric  acid, 
and  test,  after  evaporation,  in  the  usual  manner. 

EXPERIMENTS. 

a.  Apply  the  reagent,  in  the  manner  indicated,  to  solutions  of  pure  Zn  salts  of 
different  degrees  of  concentration. 

b.  Try  in  turn  pure  salts  of  Cd,  Cu,  Co,  Ni,  Ag  and  Pb. 

c.  To  a  Zn  solution  add  a  very  little  Cd  and  test.  Repeat  the  experiment, 
using  more  Cd. 

d.  In  like  manner  try  mixtures  of  Zn  and  Cu;  Zn  and  Co;  Zn  and  Ni;  Zn 
and  Fe;  Zn  and  Mg;  Zn  and  Al;  Zn  and  Pb;  Zn  and  Ag. 

e.  Then  try  more  complex  mixtures,  as,  for  example:  Zn,  Cd  and  Cu;  Zn,  Cd 
and  Co;  Zn,  Cu  and  Co;  etc. 

In  each  case  prepare  several  slides  under  different  conditions  and  note  well  the 
changes  in  the  appearance  in  the  crystals  which  separate. 


B.   By  Means  of  Primary  Sodium  Carbonate. 

Apply  a  large  drop  of  a  saturated  solution  of  the  reagent 
by  Method  /,  page  251,  to  a  neutral  or  very  slightly  acid  drop  of 
the  material  to  be  tested. 

An  amorphous  precipitate  of  what  is  doubtless  a  basic  car- 
bonate of  zinc  is  usually  at  first  formed  and  may  persist  unless 
the  reagent  is  in  large  excess;  in  the  latter  case,  after  a  few  min- 
utes, a  double  carbonate  of  zinc  and  sodium  separates  at  the 
periphery  of  the  drop.  The  crystals  of  this  salt  are  constant  and 
peculiar  to  zinc.  No  other  element  yields  compounds  of  like 
appearance.  The  salt  has  the  formula  3  Na2COr8  ZnC03-8  HoO 
(Deville).  It  takes  the  form  of  tiny  colorless  triangles  and  tet- 
raheda  or  three-pointed  or  five-pointed  agglomerates  or  rarely 
short  stout  prisms  with  pointed  ends.  The  characteristic  form 
upon  which  to  base  a  decision  are  the  triangles  or  tetrahedra. 
The  crystals  cling  tenaciously  to  the  glass,  rendering  decantation 
easy.  After  the  removal  of  the  mother  liquor  the  double  car- 
bonate can  be  dissolved  in  acid  and  subjected  to  other  tests. 


312  ELEMENTARY  CHEMICAL  MICROSCOPY 

It  is  unfortunate  that  this,  which  is  one  of  the  most  character- 
istic as  well  as  delicate  of  the  microchemical  tests  for  zinc,  should 
be  open  to  many  difficulties.  The  chief  of  these  lies  in  the  fact 
that  many  elements  are  precipitated  as  carbonates,  and  that 
these  often  bulky  precipitates  interfere  with  or  mask  the  zinc 
reaction.  Among  the  interfering  elements,  those  most  frequently 
met  with  are  doubtless  calcium,  strontium,  barium,  magnesium, 
cadmium,  lead,  iron,  manganese,  cobalt  and  nickel.  Of  this  list, 
calcium,  strontium,  barium  and  lead  will  probably  have  been  re- 
moved by  previous  treatment  with  sulphuric  acid.  Zinc  may 
be  separated  from  the  remaining  elements  of  this  list  by  treating 
with  ammonium  hydroxide  and  hydrogen  peroxide  and  finally 
extracting  with  a  drop  or  two  of  moderately  concentrated  sodium 
hydroxide  solution.  To  this  clear  extract  primary  sodium  car- 
bonate is  added. 

Schoorl  has  pointed  out  that  the  best  results  are  to  be  obtained 
from  acetic  acid  solutions  of  zinc  to  which  normal  sodium  car- 
bonate is  added.  This  method  is  unquestionably  the  best  in  the 
analysis  of  complex  mixtures  and  when  the  per  cent  of  zinc 
present  is  low.  The  Behrens  method  of  direct  addition  of  pri- 
mary carbonate  is  restricted  to  simple  salts  of  zinc  or  to  mix- 
tures known  to  contain  no  interfering  elements. 

If  only  a  very  small  amount  of  cadmium  is  present,  it  is  pre- 
cipitated before  the  zinc,  and  by  avoiding  the  addition  of  an 
excess  of  the  reagent,  decanting  the  clear  liquid  and  adding  to 
the  decanted  liquid  a  fresh  portion  of  the  reagent  in  sufficient 
quantity,  the  zinc  can  be  precipitated  as  the  double  carbonate. 
When  considerable  cadmium  is  present  this  method  is  not  feasible. 
In  such  an  event  recourse  may  be  had  to  ammoniacal  solutions,  as 
suggested  by  Behrens.  The  test  drop  is  made  strongly  ammonia- 
cal and  to  it  primary  sodium  carbonate  is  added.  Cadmium  is 
immediately  precipitated,  while  the  zinc  remains  in  solution. 
The  clear  solution  is  decanted  at  once.  After  a  few  seconds 
zinc  separates  from  the  decanted  solution  as  the  double  car- 
bonate in  the  forms  described  above.  Some  little  skill  and 
experience  is  generally  necessary  in  order  to  obtain  good 
results. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       313 

Precautions. 

Salts  of  ammonium  must  be  absent  or  present  only  in  small 
amounts. 

The  separation  of  typical  crystals  is  always  slow  and  cannot 
safely  be  hastened. 

It  is  essential  that  an  excess  of  the  reagent  be  employed. 
Failure  not  infrequently  results  from  a  neglect  of  this  precaution. 
This  is  particularly  true  if  the  test  drop  is  acid.  Because  of  the 
necessity  of  adding  large  amounts  of  primary  sodium  carbonate, 
the  test  drop  must  be  of  greater  volume  than  is  usual  in  micro- 
chemical  testing. 

EXPERIMENTS. 

a.  Try  precipitating  Zn  in  acid,  neutral  and  ammoniacal  solutions. 

b.  Test  mixtures  of  Zn  and  Cd,  first  in  neutral,  and  then  in  ammoniacal  solu- 
tions. 

c.  Experiment  with  Zn  in  the  presence  of  the  interfering  elements  noted  above. 


C.   By  Means  of  Oxalic  Acid. 

The  reagent  is  applied  by  Method  7,  page  251;  see  Cal- 
cium, Method  B,  page  291,  Strontium,  Method  B,  Barium, 
Method  B,  pages  295  and  298. 

Zinc  yields  ZnC204  •  2  H20  as  small  double  spherulites,  as 
pseudo-octahedra  singly  or  united  in  twos,  and  as  thin  rhombs. 
The  great  majority  of  the  crystals  separating  usually  have  their 
angles  rounded.  It  is  rare  that  a  preparation  is  obtained  giving 
clear-cut  crystals. 

These  crystals,  when  examined  with  a  low  power,  often  bear  a 
striking  resemblance  to  the  oxalates  of  calcium  and  strontium; 
therefore  to  avoid  error  the  alkaline  earths  should  first  be  re- 
moved. 

Cadmium  gives  clear  colorless  monoclinic  prisms  and  tabular 
crystals  of  the  formula  CdC204  •  3  H20.  The  prisms  are  usually 
very  long  and  show  a  marked  tendency  to  form  large  X's,  and 
radiating  aggregates.  From  concentrated  solutions  octahedral 
crystals  are  also  obtained.     The  typical  prisms  of  cadmium  oxa- 


314  ELEMENTARY   CHEMICAL  MICROSCOPY 

late  are  seen  only  when  working  with  comparatively  pure  salts. 
In  the  presence  of  cadmium  the  oxalic  acid  test  for  zinc  is  un- 
reliable. 

Magnesium  salts  must  be  absent,  for  under  certain  conditions 
a  double  magnesium-zinc  oxalate  in  hexagons  and  more  or  less 
irregular  plates  will  separate. 

From  a  number  of  other  precipitated  oxalates,  zinc  oxalate 
may  be  separated  by  dissolving  it  in  ammonium  hydroxide  and 
decanting  from  the  insoluble  precipitate.  Upon  evaporation 
the  ammoniacal  solution  will  deposit  zinc  oxalate,  but  no  longer 
in  the  typical  form  described  above,  but  as  masses  of  radiating 
curving  needles.  Unfortunately  this  method  is  not  applicable 
in  the  presence  of  magnesium  and  cadmium. 

Precautions. 

The  solution  to  be  tested  should  be  neutral  or  only  slightly 
acid,  and  rather  concentrated  with  respect  to  zinc. 

Lead,  silver,  copper,  cobalt,  nickel,  iron,  aluminum,  manganese 
and  chromium  interfere  with  the  detection  of  zinc  by  means  of 
oxalic  acid.  They  should  first  be  removed  if  reliable  results  are 
to  be  obtained. 

As  stated  above,  zinc  oxalate  may  be  confused  with  the  oxa- 
lates of  calcium  and  strontium,  while  magnesium  and  barium 
seriously  modify  its  characteristic  appearance. 

EXPERIMENTS. 

a.  Test  a  pure  salt  of  Zn  in  dilute  and  in  concentrated  solution.  Repeat  the 
experiments,  substituting  Cd  for  the  Zn. 

b.  Make  a  preparation  of  ZnC204  •  2  H20;  draw  off  the  supernatant  liquid,  add 
NH4OH;  warm  gently  and  study  the  preparation.  Prepare  slides  of  different 
degrees  of  concentration. 

c.  Recrystallize  CdC204  •  3  H20  in  the  same  manner  as  the  Zn  salt. 

d.  Test  mixtures  of  Zn  and  Cd. 

e.  Recrystallize  the  mixed  oxalates  from  NH4OH. 

/.  Make  mixtures  of  Zn  and  the  interfering  elements  listed  above.  Treat  the 
precipitated  oxalates  with  NH4OH.     Then  try  Cd  in  the  same  manner. 

g.  Try  precipitating  Zn  with  HKC204;  K2C204;  (NH4)2C204.  Then  try  Cd  in 
like  manner. 


MICROCHEMICAL  REACTIONS  OF  THE  COxMMON  ELEMENTS       315 

D.   By  Means  of  Sodium  Nitroprusside.1 

Apply  the  reagent  by  Method  /,  page  251,  to  a  neutral 
or  slightly  acid  solution. 

Zinc  yields  a  nitroprusside  of  low  solubility  in  the  form  of 
spherical  grains,  botryoidal  masses  or  tiny  circular  disks  of  a  very 
faint  brownish  color.  Upon  standing,  a  large  number  of  distinct 
faces  develop  upon  the  spheres  (combination  of  cube  and  dodeca- 
hedron ?).  These  crystals  are  isotropic.  The  formula  of  the 
compound  has  not  yet  been  established;  that  of  the  reagent  can 
be  written  Na2  •  NO  •  Fe  (CN)5  •  2  H20.  If  the  zinc  merely  re- 
places the  sodium,  we  should  obtain  Zn  •  NO  •  Fe  (CN)5  •  xH20,  or, 
on  the  other  hand,  we  may  be  dealing  with  a  sodium-zinc  nitro- 
prusside. In  the  presence  of  free  mineral  acids  there  is  a  ten- 
dency for  zinc  nitroprusside  to  separate  in  tiny  squares  and  stout 
prisms  or  in  fusiform  rods. 

A  moderate  amount  of  free  mineral  acid  does  not  appear  to 
prevent  the  reaction  but  retards  the  appearance  of  the  crystals. 
Much  acetic  acid  (or  acetates)  retards  the  separation  even  more. 
Heat  hastens  the  reaction,  but  warming  does  not  appear  to  be  of 
value  in  obtaining  a  better  development  of  the  crystal  form. 

Cadmium  yields  tiny  rough  globulites,  octahedra  with  rough, 
corrugated  or  even  bristling  faces,  and  drusy  masses.  Cadmium 
nitroprusside  polarizes  strongly  and  the  largest  of  the  crystals 
exhibit  brilliant  polarization  colors. 

Mixtures  of  zinc  and  cadmium  yield  rough  globulites,  most  of 
them  anisotropic. 

Manganous  salts  give  globulites  similar  in  all  respects  to  those 
obtained  with  zinc;  they  appear  later  and  rarely  develop  to  as 
large  a  size  or  exhibit  the  many  faces.  Like  the  zinc  salt  they 
are  isotropic.  In  ordinary  routine  analysis  it  is  practically  im- 
possible to  distinguish  between  zinc  and  manganese. 

Copper  yields  an  immediate  amorphous  pale  blue  precipitate. 
Often  this  shows  a  tendency  toward  the  formation  of  star-like 
skeleton  crystals.  Mixtures  of  copper  and  zinc  yield,  in  addition 
to  an  amorphous  precipitate,  the  spherical  grains  of  the  zinc  salt, 
but  in  this  case  there  is  a  tendency  toward  spherulites,  tiny 

1  Bradley,  Am.  J.  Sci.,  22  (1906),  326. 


316  ELEMENTARY   CHEMICAL   MICROSCOPY 

bristling  masses  and  tiny  crosses  and  stars,  closely  resembling 
the  forms  obtained  with  cadmium.  They  differ,  however,  from 
the  cadmium  salt  in  that  they  do  not  polarize. 

Nickel  gives  a  light  green  amorphous  precipitate;  cobalt  a 
similar  pink  one;  while  iron,  if  heated,  yields  a  yellow  deposit. 

Mercurous  salts  (nitrate)  give  a  gelatinous  amorphous  mass 
of  a  yellowish  tint. 

Mercuric  salts  and  those  of  silver,  lead,  tin,  antimony,  bismuth, 
aluminum,  magnesium  and  the  alkaline  earths  appear  to  give 
no  precipitates  and  to  yield  no  crystals  even  in  concentrated 
solution  or  upon  evaporation. 

Precautions. 

The  solution  should  be  neutral  or  but  faintly  acid  and  should 
be  moderately  concentrated  with  respect  to  zinc. 

If  no  result  is  obtained  upon  the  first  test,  make  a  second, 
employing  a  considerably  greater  amount  of  the  unknown 
substance. 

Heating  the  preparation  hastens  the  reaction. 

If  a  precipitate  is  obtained,  zinc,  cadmium,  copper,  nickel, 
cobalt,  iron  or  manganese  are  present  and,  conversely,  if  no  pre- 
cipitate appears,  these  elements  must  be  absent. 

Sodium  nitroprusside  is  thus  a  convenient  group  reagent. 

EXPERIMENTS. 

a.  Try  the  reagent  upon  several  different  concentrations  of  Zn. 

b.  Try  with  Cd,  then  with  mixtures  of  Zn  and  Cd. 

c.  Try  salts  of  Cu,  Ni,  Co,  Mn,  first  as  pure  salts,  then  as  mixtures  with  Zn. 


CADMIUM. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 
of  Cadmium. 

A.  ISOTROPIC. 

B.  ANISOTROPIC. 

Hexagonal.  —  Iodide,  ammonium-cadmium  bro- 
mide; ammonium-cadmium  chloride;  potas- 
sium-cadmium chloride. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       317 

Tetragonal. 

Orthorhombic.  —  Bromide. 

Monoclinic. —  Acetate;  chloride;  sulphate. 

Triclinic. 

DETECTION. 

A.   By  Means  of  Ammonium  Mercuric  Sulphocyanate. 
Read  Method  A,  Zinc,  page  307. 

The  prismatic  crystals  of  Cd  (CNS)2  •  Hg(CNS)2  are,  in  a 
similar  manner  to  the  zinc  salt,  colored  a  faint  chocolate  brown 
by  traces  of  copper.  This  brown  color  intensifies  with  an  in- 
crease in  the  amount  of  copper.  When  considerable  copper  is 
present,  the  copper  double  salt  first  separates,  since  it  is  slightly 
less  soluble  than  the  cadmium  compound;  then  mixed  crystals 
form,  in  which  the  copper  apparently  predominates  over  the 
cadmium.  These  mixed  crystals  are  of  a  deep  bluish  green 
color.  By  this  time  most  of  the  copper  and  but  little  of  the 
cadmium  have  been  precipitated,  and  the  concentration  has  also 
reached  such  a  point  that  the  cadmium  double  salt  begins  to 
separate  in  the  crystal  forms  described  on  page  309.  These 
are,  however,  still  mixed  crystals,  for  they  are  colored  brown 
by  the  small  amount  of  copper  still  in  solution. 

As  in  the  case  of  the  zinc  reaction,  iron  may  sometimes  color 
the  cadmium  salt  a  reddish  brown. 

Cobalt  colors  the  cadmium  salt  blue.  Much  cobalt  gives  an 
intense  blue  color  and  alters  the  crystal  form. 

Magnesium  and  aluminum  have  even  less  effect  than  in  the 
case  of  zinc. 

Before  testing  for  cadmium  with  the  sulphocyanate  reagent, 
it  is  best  to  first  remove  any  lead  or  silver  which  may  be  present. 

If  a  small  amount  of  zinc  is  also  present,  mixed  crystals  con- 
taining zinc  and  cadmium  first  separate,  whose  crystal  form  can 
be  described  as  non-feathery  skeletons;  soon  after  this  the 
cadmium  double  salt  separates  in  its  typical  form.  In  order 
that  this  sequence  shall  be  brought  about,  it  is  best  to  employ  a 
solution  somewhat  more  dilute  than  when  zinc  is  known  to  be 
absent.     Much  zinc  usually  prevents  the  formation  of  any  of  the 


318  ELEMENTARY   CHEMICAL   MICROSCOPY 

prismatic  crystals  of   the   cadmium  salt,   only  mixed   crystals 
resulting. 

Precautions. 

Cadmium  salts  of  the  organic  acids,  as,  for  example,  cadmium 
acetate,  fail  to  yield  a  satisfactory  test.  It  is  therefore  best  to 
evaporate  the  unknown  with  nitric  acid  and  drive  off  the  excess 
of  acid  before  adding  the  sulphocyanate  reagent.  It  follows  that 
the  addition  of  sodium  or  ammonium  acetate  to  very  acid  solu- 
tions to  lessen  the  effect  of  the  mineral  acid  is  in  this  case  unwise. 
It  is  better  to  evaporate  to  dryness. 


B.   By  means  of  Oxalic  Acid. 
Read  Method  C,  Zinc,  page  313. 

The  typical  crystals  of  cadmium  oxalate  CdC204  •  3  H20  con- 
sist of  long,  clear,  colorless,  monoclinic  prisms,  singly,  in  X's, 
or  in  clusters.  The  obliquely  truncated  ends  constitute  a  dis- 
tinctive feature. 

Manganous  oxalate  MnC204  •  3  H20  separates  in  groups  of 
radiating  prisms,  which  the  careless  observer  sometimes  con- 
fuses with  the  cadmium  salt  or  vice  versa.  The  ends  of  the 
prisms  of  the  two  salts  are  quite  different  however  in  appearance. 


C.   By  Means  of  Sodium  Nitroprusside. 
See  Zinc,  Method  D,  page  315. 

MERCURY. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 
of  Mercury. 

A.  ISOTROPIC. 

B.  ANISOTROPIC. 

Hexagonal. 

Tetragonal.  —  Mercurous  bromide,  chloride    and 

iodide;    mercuric    cyanide;    red   mercuric 

iodide. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       319 

Orthorhombic.  —  Mercuric  bromide;  mercuric  chlo- 
ride; yellow  mercuric  iodide. 
Monoclinic.  —  Mercurous  and  mercuric  nitrates. 
Tridinic. 

DETECTION. 

A.   As  Metallic  Mercury  by  Sublimation. 

Heat  upon  a  piece  of  platinum  foil  or  upon  a  glass  slide  a 
little  anhydrous  sodium  carbonate  until  all  the  moisture  it  con- 
tains has  been  expelled,  cool,  powder  and  mix  a  very  small 
amount  with  a  little  of  the  material  to  be  examined  —  transfer 
to  a  small  tube  of  hard  glass  not  over  2  millimeters  in  internal 
diameter,  thin-walled  and  sealed  at  one  end.  Jar  the  mixture 
down  so  as  to  obtain  clean  walls.  Heat  gently  over  the  flame 
of  a  Bunsen  burner  turned  down  to  a  flame  1  centimeter  high. 
The  mercury  compound  will  be  decomposed  and  tiny  globules  of 
metallic  mercury  will'  condense  upon  the  walls  of  the  tube. 
Examine  under  the  microscope.  With  a  stiff  hair  or  glass  rod 
drawn  down  to  a  hair  gently  rub  the  ring  of  sublimate.  Examine 
again.     The  mercury  will  have  united  into  larger  globules. 

Introduce  into  the  tube  two  or  three  small  fragments  of  iodine. 
Then  insert  the  open  end  of  the  tube  into  a  piece  of  cork ;  warm 
the  iodine  very  gently  and  set  the  tube  aside  for  a  few  minutes. 
Yellow  and  red  mercuric  iodide  will  be  formed.  Warming  again 
will  hasten  the  reaction  and  cause  the  sublimation  of  some  of  the 
mercuric  iodide.  Rectangular  and  rhombic  plates  and  dendritic 
masses  of  both  the  vermilion  colored  iodide  and  the  yellow 
modification  will  be  obtained. 

No  other  known  element  gives  a  reaction  even  remotely  re- 
sembling this  one. 

From  large  volumes  of  liquid  the  mercury  may  be  removed  by 
acidifying  with  hydrochloric  acid  and  dropping  in  a  steel  needle 
around  which  has  been  wound  a  tiny  spiral  of  thin  gold  foil.  The 
deposited  mercury  amalgamates  with  the  gold.  The  electrolytic 
couple  is  lifted  out  after  some  time,  washed,  the  gold  foil  removed, 
dried,  placed  in  a  subliming  tube  and  the  mercury  expelled  by 
heating.     The  sublimate  is  then  characterized  as  above. 


320  ELEMENTARY   CHEMICAL   MICROSCOPY 

From  drops  containing  moderate  amounts  of  mercury,  the 
metal  may  be  separated  by  a  fragment  of  magnesium,  or  it  may 
be  deposited  upon  a  bit  of  copper.  If  in  the  latter  case  the  spot 
of  deposit  be  rubbed  it  becomes  silvery  white.  If  the  coated 
copper  is  placed  in  a  subliming  tube  and  heated  the  mercury 
will  be  volatilized  and  will  condense  in  characteristic  globules. 

EXPERIMENTS. 

a.  Test  several  mercurous  and  mercuric  salts  by  heating  them  with  Na2C03. 
Examine  the  sublimates.  Rub  them  gently  with  a  hair-like  glass  rod  and  note 
that  the  globules  unite. 

b.  Obtain  a  deposit  of  Hg  upon  a  tiny  bit  of  Cu  foil —  i  millimeter  by  3  milli- 
meters—  by  heating  in  a  drop  of  a  solution  of  an  Hg  salt  acidified  with  HO. 
Dry  and  sublime. 

c.  Introduce  a  fragment  of  iodine  in  one  or  more  of  the  tubes,  warm  gently  and 
allow  to  stand  about  five  minutes.     Examine  for  crystals  of  Hgl2. 


B.  Differentiating  between  Mercurous  and  Mercuric  Salts. 
Add  Hydrochloric  Acid.  —  With  mercuric  salts  there  is  no 
precipitation.  Mercurous  salts  give  an  immediate  amorphous 
precipitate  of  a  white  chloride  HgCl.  Under  unusual  conditions 
and  exceedingly  dilute  solutions,  mercurous  chloride  may  some- 
times be  obtained  in  the  form  of  slender  needles.  To  charac- 
terize the  white  precipitate,  draw  off  the  supernatant  solution 
and  add  to  the  residue  a  drop  of  dilute  ammonium  hydroxide.  A 
black  compound  of  the  formula  NH2Hg2Cl  is  immediately  formed. 
Examined  with  a  \  inch  or  an  8  millimeter  objective  the  black 
compound  is  seen  to  consist  of  a  mass  of  tiny  acicular  crystals, 
tiny  squares,  crosses  and  fusiform  grains. 

EXPERIMENTS. 

a.  Precipitate  HgCl,  examine  with  the  microscope. 

b.  Add  NH4OH  to  the  white  precipitate  and  examine  again. 


Add  Potassium  Bichromate  and  Nitric  Acid.  —  To  the  drop  to 
be  tested  add  nitric  acid.  Place  nearby,  a  drop  of  solution  of 
bichromate.     Warm  the  drops  over  the  micro-flame  and  while 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       321 

hot  cause  the  bichromate  to  flow  into  the  test  drop.  Mercurous 
salts  yield  characteristic  crystals.     Mercuric  salts  do  not.1 

There  are  generally  formed  with  mercurous  salts  a  number  of 
different  compounds.  There  first  separates  a  dark  red  granular 
precipitate,  soon  changing  into  dark  red  crosses,  bundles  of  irregu- 
lar crystals  and  peculiar  dendrites  and  skeleton  masses.  Later 
yellow  crystallites  appear. 

In  any  given  test  the  appearance  of  the  precipitate  both  as 
to  crystal  form  and  color  will  depend  upon  the  concentration  of 
the  drops,  the  degree  of  acidity  and  the  temperature. 

Mercuric  salts  give  no  such  precipitates  and  no  crystalline 
compounds  will  appear  unless  the  preparation  is  allowed  to 
evaporate  practically  to  dryness.  There  will  then  appear  light 
yellow  feathery  dendritic  and  radiating  branching  moss-like 
masses. 

Lead  yields  slender  yellow  monoclinic  prisms,  seldom  grouped 
in  masses.  This  element  unless  present  in  excess  does  not  appear 
to  seriously  interfere  with  the  test  for  mercury. 

Silver  separates  in  dark  red  pleochroic  plates  and  scales  which 
may  often  mask  the  mercury  compounds. 

EXPERIMENTS. 

Test  as  above  both  mercurous  and  mercuric  salts  with  and  without  HNO3 
present  in  both  cold  and  hot  solutions. 


C.  Add  to  a  Drop  of  the  Material  a  Tiny  Fragment  of  Potas- 
sium Iodide.  —  See  Method  777,  page  252.  Mercuric  salts  yield 
vermilion  colored  mercuric  iodide;  mercurous  salts  a  heavy  bright 
yellow  amorphous  precipitate  somewhat  resembling  lead  iodide 
in  color  but  instead  of  being  in  plates  always  agglutinated  in  a 
formless  mass. 

With  mercuric  salts  we  obtain  one  of  the  best  and  most  satis- 

1  Bichromate  added  to  hot  unacidified  HgCl2  solutions  causes  the  separation 
on  cooling  of  hard  star-like  masses  of  crystals.  According  to  Millon  (Ann.  chim. 
phys.  (3)  18,  388)  this  compound  has  the  formula  HgCl2  •  K2Cr207.  Ammonium 
bichromate  gives  orthorhombic  six-sided  prisms  of  the  compound  HgCl2-3(NH«)i 
Cr207. 


322  ELEMENTARY   CHEMICAL  MICROSCOPY 

factory  tests  for  mercury.  At  the  moment  the  potassium  iodide 
strikes  the  drop  a  white  or  pinkish  cloud  appears,  rapidly  chang- 
ing to  yellow  then  to  brilliant  red.  The  mercuric  iodide  Hgl2 
first  formed  is  very  soluble  in  excess  of  the  reagent  forming  the 
soluble  compound  HgL  •  2  KI.  The  precipitate  therefore  appears 
as  an  ever-widening  circle  about  the  fragment  of  solid  reagent 
until  the  latter  is  completely  dissolved.  If  the  outer  edge  of  the 
brilliant  red  circle  is  now  examined  with  a  moderately  high  power 
it  will  be  seen  to  consist  of  tiny  ruby  red  rhombs  and  rods  to- 
gether with  more  or  less  spherical  masses  and  imperfect  rosettes. 
Precautions  must  be  taken  to  avoid  adding  an  excess  of  reagent; 
otherwise  no  permanent  separation  will  take  place.  In  order  to 
avoid  the  possibility  of  error  it  is  always  well  to  add  a  fragment 
of  copper  sulphate,  which  will  take  up  the  excess  of  iodide  and 
cause  the  separation  of  the  mercuric  salt. 

EXPERIMENTS. 

See  under  Lead,  Method  A.  page  325. 


D.  Mercuric  Salts  can  be  detected  through  the  Formation  of 
Double  Sulphocyanates. 

This  test  is  the  reverse  of  that  employed-  for  the  detection  of 
Zinc  (Method  A ,  page  307) ;  of  Copper  (Method  A ,  page  339) ; 
or  of  Cobalt  (Method  A,  page  366),  to  which  the  student  is  re- 
ferred for  details. 

Add  to  a  small  test  drop  (which  must  not  contain  much  free 
mineral  acid)  a  fragment  of  ammonium  sulphocyanate  about  the 
size  of  a  pinhead.  Stir  until  dissolved.  Place  next  this  drop  a 
tiny  drop  of  water  in  which  is  dissolved  a  very  little  zinc  sul- 
phate. Cause  the  test  drop  to  flow  into  the  zinc  solution.  Char- 
acteristic crystals  of  zinc-mercury  sulphocyanate  will  appear. 

Instead  of  zinc  sulphate,  copper  sulphate  or  cobalt  nitrate 
may  be  employed. 

With  simple  mixtures,  this  test  is  a  very  beautiful  one,  but 
with  complex  material  it  is  sometimes  difficult  to  adjust  the  con- 
ditions especially  as  regards  the  quantity  of  ammonium  sulpho- 
cyanate required. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       323 

EXPERIMENTS. 

a.  Test  as  above  HgClz,  using  ZnS04. 

b.  Try  again,  this  time  introducing  a  trace  of  CuS04. 

c.  Try  this  test  with  CuSO<  but  with  no  ZnS04  present  (which  method  is  most 
satisfactory?). 


LEAD.1 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 
of  Lead. 

A.  ISOTROPIC.  —  Nitrate  (I). 

B.  ANISOTROPIC. 

Hexagonal.  —  Iodide. 
Tetragonal. 

Orthorhombic.  —  Bromide;  chloride;2  sulphate;  tar- 
trate. 
Monoclinic. — Acetate;  chromate;  sulphocyanate. 
Triclinic. 


DETECTION. 


A.   By  Means  of  Potassium  Iodide. 

Apply  the  reagent,  by  Method  III,  page  252,  to  the  test 
drop  slightly  acidified  with  nitric  acid. 

Lead  iodide  Pbl2  is  at  once  formed  as  a  bright  yellow  precipi- 
tate in  a  circular  band  about  the  reagent  fragment.  The  circle 
gradually  becomes  larger  and  larger  and  at  its  outside  circum- 
ference beautiful  hexagonal  plates  appear.  These  plates  and 
flakes  of  lead  iodide  appear  greenish  or  brownish  yellow  by 
transmitted  light,  sometimes  even  gray,  according  to  their  thick- 
ness. By  reflected  light  lead  iodide  plates  glow  and  glisten  and 
display  the  iridescent  colors  of  thin  films,  an  extremely  charac- 
teristic feature  of  this  salt. 

These  hexagons  of  lead  iodide  do  not  belong,  according  to 

1  Lead,  silver  and  copper  are  introduced  at  this  point  rather  than  in  their 
proper  position  in  the  Periodic  System  because  of  their  close  relations  in  qualita- 
tive analysis. 

2  Recrystallized  from  hot  water  PbCl2  is  pseudohexagonal. 


324  ELEMENTARY   CHEMICAL   MICROSCOPY 

Behrens,  to  the  hexagonal  system,  as  usually  stated,  but  are 
probably  only  pseudohexagonal  and  in  reality  orthorhombic. 

From  neutral  solutions  containing  lead  in  the  form  of  lead 
acetate,  potassium  iodide  will  generally  precipitate,  in  addition 
to  the  normal  iodide,  basic  iodides  of  variable  composition,  such 
as  Pbl2  •  PbO;  Pbl2  •  2  PbO  (?). 

Lead  iodide  can  be  recrystallized  from  hot  water,  best  if  acidi- 
fied with  nitric  acid.  On  cooling,  large,  beautifully  formed 
hexagons  separate.  A  large  drop  of  water  is  necessary  in  order 
that  good  results  may  be  obtained. 

Heated  with  hydrochloric  acid  lead  iodide  dissolves,  and  on 
cooling  crystals  of  the  normal  iodide  PbL,  the  normal  chloride 
PbCl2  and  a  chloriodide  PbCl2  •  Pbl2  or  2  PbCl2  •  Pbl2  (or  both) 
will  separate.  The  chloriodides  appear  in  the  form  of  needles 
of  a  faint  yellow  color. 

Silver  iodide  separates  as  a  yellowish  amorphous  mass  insoluble 
in  hot  water  and  in  hot  nitric  acid. 

Mercuric  iodide  takes  the  form  of  red  rhombs.  Mercurous 
salts  acidified  with  nitric  acid  usually  give  in  addition  to  the 
heavy  precipitate  of  mercurous  iodide  the  ruby  colored  rhombs 
-of  the  mercuric  salt. 

If  cuprous  salts  are  present  a  white  granular  precipitate  of 
cuprous  iodide  is  formed  and  iodine  is  set  free.  Cupric  salts 
will  behave  similarly. 

Thallium  is  precipitated  as  an  exceedingly  fine  granular  pre- 
cipitate. 

Antimony  and  bismuth  salts  interfere  with  the  reaction  for 
lead.  These  elements  yield  with  potassium  iodide,  double 
iodides  which  separate  in  neat,  well-formed  crystals.  Solutions 
containing  lead,  antimony  and  bismuth,  when  treated  with 
potassium  iodide,  yield  a  dark  reddish  brown,  sandy  precipitate 
wholly  unlike  in  appearance  anything  obtained  with  the  different 
elements  alone.  Boiling  the  mixed  product  with  water  will 
generally  cause  a  partial  decomposition,  and  on  cooling  hexagons 
and  irregular  plates  of  lead  iodide  will  appear.  In  the  presence 
of  a  little  bismuth,  lead  iodide  separates  as  orange  red  disks  and 
plates,  or  the  iodide  scales  may  even  appear  crimson  in  color. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       325 

Precautions. 

An  excess  of  the  reagent  must  be  avoided,  otherwise  the  pre- 
cipitate at  first  formed  will  be  dissolved  because  of  the  formation 
of  a  double  iodide  of  the  composition  Pbl2  •  2  KI  •  x\i20}  Not 
infrequently  colorless  crystals  of  this  double  iodide  will  be  seen 
in  the  immediate  neighborhood  of  the  reagent  fragment.  The 
addition  of  a  drop  of  water  will  usually  cause  the  decomposition 
of  the  double  salt  and  a  precipitation  of  the  normal  iodide. 

Double  iodides  of  lead  with  many  elements  are  known,  most 
of  them  crystallizing  readily,2  but  it  is  not  often  that  there  will  be 
a  sufficient  separation  of  these  interesting  salts  to  interfere  in 
any  way  with  the  detection  of  lead. 

Too  much  nitric  acid  in  the  water  employed  for  recrystallizing 
the  precipitate  of  lead  iodide  will  cause  partial  decomposition 
and  consequently  the  separation  of  colorless  octahedra  of  lead 
nitrate. 

EXPERIMENTS. 

a.  To  a  test  drop  containing  Pb(N03)2  add  KI.  Study  the  preparation,  then 
add  a  drop  of  water  and  heat  to  boiling.  After  the  drop  has  cooled,  study  it 
again.  Repeat  the  experiment,  but  this  time  use  an  excess  of  KI.  Try  again  in 
acidified  solutions. 

b.  In  like  manner  test  a  preparation  of  Pb(C2H302)2. 

c.  Make  a  preparation  of  PbS04.  Decant  the  mother  liquor,  add  to  the  sul- 
phate residue  a  drop  of  water,  acidify  with  HN03,  then  add  a  fragment  of  KI. 
After  a  few  seconds  examine  the  preparation. 

d.  Make  a  mixture  of  Pb  and  Ag,  test  with  KI.  Then  try  in  turn  mixtures  of 
Pb  and  Sb;  Pb  and  Bi;  Pb,  Sb  and  Bi;  Pb  and  Cu;  Pb  and  Sn. 

e.  Test  a  preparation  of  HgCl2.  Then  one  of  Hg(N03)2.  Make  a  mixture  of 
Pb(N03)2  and  Hg(N03)2  and  test. 


B.   By  Means  of  Hydrochloric  Acid. 

Apply  the  reagent  by  Method  77/,  page  252,  to  the  test 
drop  acidulated  with  a  little  nitric  acid. 

This  method  of  adding  the  reagent  is  not  so  good  as  allowing 
two  drops  to  flow  together  but  is  adopted  so  as  to  conform  to  that 
for  testing  for  silver  and  mercury. 

1  Brooke,  Ch.  N.,  1898,  191. 

2  See  Mosnier,  Ann.  chim.  phys.  (7)  12,  374;  Comptes  rend.,  120,  444. 


326  ELEMENTARY   CHEMICAL   MICROSCOPY 

Lead  chloride  PbCl2  separates  at  once  in  the  form  of  char- 
acteristic, white,  long  acicular  crystallites  belonging  to  the  ortho- 
rhombic  system.  There  are  also  seen  feathery  dendritic  X's  and 
long  irregular  ragged  prisms. 

The  appearance  of  the  lead  chloride  separating  varies  with  the 
concentration  of  the  solution  being  tested  and  with  the  nature 
of  the  substances  present.  If  the  test  drop  is  not  sufficiently 
concentrated  the  lead  chloride  will  not  separate  at  once  in  the 
form  of  the  characteristic  crystallites,  but  will  appear  more 
slowly,  prismatic  forms  being  the  rule.  This  question  of  con- 
centration becomes  a  most  important  one  if  the  substance  con- 
tains salts  with  which  lead  chloride  can  unite  to  form  double 
salts,  as  for  example  chlorides  of  the  alkali  metals  and  ammonium, 
for  in  such  an  event  dilute  or  even  moderately  concentrated 
drops  fail  to  yield  recognizable  forms.  Indeed  it  may  be  said 
that  testing  for  lead  with  hydrochloric  acid  is  not  advisable  in 
the  presence  of  members  of  Groups  I  and  II. 

In  neutral  solutions  of  lead  acetate  there  may  be  precipi- 
tated in  the  presence  of  members  of  Group  I  and  no  excess  of 
the  reagent,  colorless,  highly  refractive  prisms  of  the  formula 
Pb(OH)Cl  (n  =  2.08  to  2.16)  belonging  to  the  orthorhombic  sys- 
tem but  sometimes  also  appearing  as  monoclinic  prisms. 

Lead  chloride  is  slightly  more  soluble  in  water  containing  a 
little  nitric  acid  than  in  pure  water,  hence  the  separation  of  lead 
as  chloride  is  never  complete. 

Lead  chloride  differs  from  the  chlorides  of  silver  and  mercurous 
mercury  in  being  easily  soluble  in  hot  water,  thus  affording  a 
simple  method  of  separation.  On  cooling,  the  lead  chloride  no 
longer  appears  in  the  forms  stated  above  but  assumes  that  of 
thin  pseudohexagonal  prisms,  rhombs  and  hexagons. 

Recrystallized  in  the  presence  of  Group  I,  double  chlorides 
result,  which  generally  separate  more  slowly.  The  crystal  form  is 
quite  different  from  that  of  the  normal  salt.  It  is  quite  impor- 
tant that  the  student  should  be  familiar  with  at  least  the  double 
chloride  of  cesium  and  lead  (cesium  chloroplumbate),  since  this 
compound  not  infrequently  makes  its  appearance  when  testing  for 
tin  with  cesium  chloride  and  is  quite  apt  to  puzzle  the  beginner. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       327 

Alkalies  convert  lead  chloride  into  a  basic  chloride  to  which 
the  formula  PbCl2  •  3  PbO  •  4  H20  is  generally  assigned. 

Thallous  salts  yield  with  hydrochloric  acid  star-  and  cross-like 
crystallites  differing  considerably  from  those  given  by  lead. 
There  is  little  danger  of  confusing  these  two  elements,  since  re- 
crystallizing  thallous  chloride  from  hot  water,  in  which  it,  like 
lead  chloride,  is  soluble,  yields  well-formed  cubes. 

In  the  presence  of  chlorides  of  antimony  and  bismuth  complex 
chlorides  of  low  solubility  are  sometimes  formed,  against  which 
the  analyst  should  be  on  his  guard. 

Silver  gives  an  amorphous  precipitate  and  mercurous  salts  a 
fine  granular  one  without  resolvable  structure. 

EXPERIMENTS. 

a.  To  a  drop  of  a  concentrated  solution  of  Pb(N03)2  add  a  drop  of  dilute 
HC1  in  the  manner  described  above.  Make  several  other  preparations  varying 
the  concentration  of  the  test  drops. 

b.  Recrystallize  a  preparation  of  PbCl2  by  heating  to  boiling  with  a  large  drop 
of  water. 

c.  Recrystallize  a  preparation  of  PbCl2  in  the  presence  of  NaCl,  another  in  the 
presence  of  KC1;  of  NH4CI;  of  CsCl. 

d.  Test  a  solution  of  Pb  and  Sb.  Then  one  of  Pb  and  Bi.  Then  one  contain- 
ing all  three  elements. 

e.  To  a  preparation  of  PbCl2  add  a  drop  of  NH4OH. 


C.    Through  the  Formation  of  a  Triple  Nitrite  of  Lead,  Cop- 
per and  Potassium. 

To  the  moderately  concentrated  neutral  test  drop  add  a 
trace  'of  acetic  acid,  then  a  fragment  or  two  of  sodium  acetate 
and  of  copper  acetate.  Stir.  Then  add  a  fragment  of  potas- 
sium nitrite. 

There  is  formed  the  salt  K2CuPb(N02)6  as  tiny  squares  or 
rectangular  plates,  or  tiny  cubes  and  rectangular  prisms  which 
are  brown  by  reflected  light,  jet  black  by  transmitted  light.  The 
crystals  appear  to  be  isometric. 

In  this  salt  the  potassium  may  be  replaced  by  rubidium,  yield- 
ing a  compound  of  lower  solubility,  or  by  cesium  which  will  give 
a  salt  of  less  and  finally  by  thallium,  one  of  least  solubility  and 
therefore  the  test  of  highest  delicacy.     These  salts  are  probably 


328  ELEMENTARY   CHEMICAL   MICROSCOPY 

isomorphous.  The  size  of  the  crystals  obtained  decreases  as 
their  solubility  decreases. 

This  test  is  a  most  convenient  one  if  alloys  or  substances  sus- 
pected of  containing  both  lead  and  copper  are  being  examined. 
It  is  then  only  necessary  to  add  to  the  solution,  sodium  acetate, 
potassium  nitrite  and  acetic  acid.  If,  after  waiting  a  reasonable 
time,  no  triple  nitrite  separates,  cesium  chloride  or  thallous 
nitrate  can  be  added. 

The  nickel  salt  also  forms  squares,  rectangles  and  cubes  but 
these  are  light  brown  by  transmitted  light  not  black. 

Cobalt  is  immediately  precipitated  by  potassium  nitrite  as  a 
very  insoluble  double  nitrite  of  potassium  and  cobalt  in  the  form 
of  a  reddish  brown  powder,  or  in  well-defined  very  tiny  cubes  and 
octahedra. 

The  triple  nitrite  may  be  written  thus: 

2  KNOo  •  Cu(N02)2  •  Pb(N02),. 
Precautions. 

In  very  dilute  solutions  the  test  fails  unless  rubidium  or  cesium 
chlorides  are  added  because  of  the  too  great  solubility  of  the 
potassium  salt.  Concentration  may  sometimes  yield  the  typical 
black  crystals. 

The  addition  of  an  excessive  amount  of  potassium  nitrite  is 
objectionable  because  of  the  fact  that  the  triple  nitrite  is  quite 
soluble  in  solutions  of  this  reagent.  On  the  other  hand,  it  is 
essential  that  the  amount  added  be  very  slightly  in  excess  of 
that  called  for  by  theory.  It  is  therefore  necessary  to  proceed 
somewhat  cautiously.  Add  a  tiny  fragment  of  nitrite,  then 
after  waiting  a  few  moments,  if  no  crystals  appear  add  a  little 
more. 

Too  concentrated  solutions  of  lead  yield  sandy  black  precipi- 
tates requiring  recrystallization.  Recrystallization  can  be  effected 
by  adding  to  the  preparation  a  little  water,  a  trace  of  acetic 
acid  and  a  slight  excess  of  potassium  nitrite,  then  heating  the 
preparation  to  boiling.     Good  crystals  should  appear  on  cooling. 

Free  mineral  acids  must  be  absent. 

When  the  amount  of  lead  is  relatively  great  and  cesium  chloride 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       329 

is  added  to  increase  the  delicacy  of  the  reaction  a  double  chloride 
of  cesium  and  lead  is  formed  which  separates  simultaneously 
with  or  even  before  the  triple  nitrite. 

EXPERIMENTS. 

a.  Test  a  preparation  containing  Pb. 

b.  Try  another  preparation,  this  time  introducing  RbCl. 

c.  Try  again  with  CsCl. 

d.  By  a  series  of  careful  dilutions  determine  the  limit  of  the  precipitation  of 
Pb  as  the  K  salt,  the  Rb  salt  and  the  Cs  salt. 

e.  Test  a  mixture  of  Pb  and  Ni;  Pb  and  Co;  Pb  and  Ag. 


D.   By  Means  of  Metallic  Zinc. 

Apply  the  fragment  of  metal  to  the  center  of  the  drop  to 
be  tested;   see  Method  77/,  page  252. 

The  characteristic  appearance  of  the  different  metals  when 
separated  from  their  solution  by  an  element  higher  in  the  electro- 
chemical series  is  often  quite  sufficient  to  enable  the  analyst  to 
identify  it.  The  student  is  already  familiar  with  these  peculiari- 
ties through  the  experiments  performed  as  outlined  on  page  254. 

Lead  yields  beautiful  long  stiff  many  branching  more  or  less 
fern-like  dendrites,  whose  side  arms  are  usually  at  right  angles 
to  the  main  stem  or  rib.  Only  portions  of  the  formation  show 
bright  metallic  reflections.  The  chief  characteristic  of  the  "lead 
tree"  is  a  long  fairly  straight  trunk  or  rib  with  side  dendrites  of 
irregular  length. 

Of  "trees"  formed  by  other  metals  that  of  silver  most  nearly 
resembles  that  of  lead,  but  is  more  delicate,  more  branching,  with 
side  formations  at  angles  other  than  90  degrees  and  exhibits 
splendid  silvery  white  metallic  reflections. 

Tin  somewhat  resembles  silver  but  the  side  arms  of  the  "  trees" 
are  very  oblique  and  parallel  one  with  another,  that  is,  the  paral- 
lelism extends  across  the  main  axis  or  rib.  The  reduction  is 
slower  with  tin. 

None  of  the  remaining  metals  yield  long  loose  fern-like  or  tree- 
like forms.  Bismuth  gives  black  and  gray  feathery  and  mossy 
dendrites  with  sharp-pointed  ends  with  a  characteristic  curving 
tendency  of  the  ends  of  the  clusters.     The  mossy  dendrites  appear 


330  ELEMENTARY    CHEMICAL   MICROSCOPY 

jet  black  by  transmitted  light,  grayish  by  reflection,  their  growth 
is  rapid  and  vigorous,  finally  occupying  the  entire  area  of  the 
drop,  and  is  characteristic  of  bismuth.  Antimony  yields  black 
mossy  dendrites  but  rarely  feathery  or  curving;  they  appear 
more  granular  in  structure. 

Copper  separates  as  black,  compact  stout  mossy  masses  with 
somewhat  tabular  or  angular  ends. 

Cobalt  resembles  copper  somewhat  but  forms  dendrites  less 
readily.  Nickel  can  be  made  to  yield  a  crystalline  deposit  only 
with  great  difficulty;  only  small  mossy  patches  are  usually 
obtainable. 

Gold  yields  very  compact  mossy  or  granular  dendrites  and 
irregular  botryoidal  black  masses  which  soon  exhibit  the  char- 
acteristic golden  yellow  reflections  of  the  metal. 

Precautions. 

To  obtain  the  best  results,  the  solutions  should  be  practically 
neutral  or  only  very  slightly  acid,  otherwise  the  rapid  evolution 
of  hydrogen  will  cause  the  disintegration  of  the  deposited  crystal 
masses.     If  free  mineral  acid  is  present  add  sodium  acetate. 

Use  only  cold  solutions. 

Employ  only  a  very  minute  fragment  of  zinc,  otherwise  the 
area  of  metal  upon  which  deposition  can  take  place  is  so  great 
that  really  characteristic  growths  will  not  be  obtained. 

In  general  a  moderate  concentration  is  essential  to  the  forma- 
tion of  satisfactory  dendrites. 

EXPERIMENTS. 

If  a  number  of  elements  have  not  already  been  tested  under  Method  7/7,  page 
252,  try  a  fragment  of  Zn  in  drops  of  solutions  of  salts  of  Pb,  Bi,  Sb,  Sn,  Cu,  Cd, 
Pt,  Au  and  Hg. 


SILVER. 


Crystal  Forms  and  Optical  Properties  of  Common  Salts  of 
Silver. 

A.   ISOTROPIC.  —  Chloride  (I);  bromide  (I);  iodide 
(IorH). 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       33 1 

B.  ANISOTROPIC. 

Hexagonal.  —  Iodide;1    secondary   arsenate;    sec- 
ondary phosphate. 
Tetragonal. 

Orthorhombic.  —  Chromate;   nitrate;    nitrite;    sul- 
phate; potassium-silver  iodide. 
Monoclinic. 
Triclinic.    —  Bichromate. 

DETECTION. 

A.   By  Means  of  Hydrochloric  Acid. 

Apply  the  reagent  by  Method  III  A,  page  254,  to  the  test 
drop  previously  acidulated  with  nitric  acid. 

If  silver  is  present  an  immediate  precipitate  should  result. 
Examine  under  the  microscope.  Silver  chloride  is  so  insoluble 
in  water  that  it  is  thrown  down  as  an  amorphous  mass.  If  the 
precipitate  is  wholly  crystalline,  either  silver  is  absent  or  else 
present  in  very  small  amount.  In  order  to  identify  silver  in  an 
amorphous  precipitate  it  is  necessary  to  recrystallize  it.  Before 
so  doing  it  is  always  advisable,  and  often  necessary,  to  first 
remove  the  solution  from  the  precipitate  and  wash  the  latter.  If 
the  hydrochloric  acid  has  been  carefully  added  and  the  drop  not 
stirred,  it  is  easy  to  draw  off  the  clear  solution  from  the  curdy, 
heavy  precipitate  of  silver  chloride.  When  the  amount  of  pre- 
cipitate is  very  small  it  is  best  to  have  recourse  to  the  centrifuge 
to  accomplish  the  separation.  After  removing  the  supernatant 
liquid,  wash  the  precipitate  once  or  twice  with  hot  water  acidified 
with  nitric  acid.  The  washed  precipitate  is  then  recrystallized 
from  concentrated  hydrochloric  acid,  or  from  ammonium  hy- 
droxide. 

To  the  precipitate  of  silver  chloride,  at  the  corner  of  a  slide, 
add  a  drop  or  two  of  concentrated  hydrochloric  acid,  and  heat 
the  preparation  over  the  micro-flame.  If  the  precipitate  is  not 
completely  dissolved,  rapidly  draw  off  the  hot  acid,  without 
exercising  any  great  care.  On  cooling,  tiny  crystals  of  silver 
chloride  separate.     Octahedral  crystals  predominate. 

1  Upon  heating,  Agl  becomes  isometric. 


33  2 


ELEMENTARY    CHEMICAL   MICROSCOPY 


To  the  washed  precipitate  add  one  or  two  drops  of  strong 
ammonium  hydroxide.  After  a  second  or  two  of  contact,  draw 
off  the  ammoniacal  solution  from  any  undissolved  precipitate. 
Do  not  heat  the  preparation.  Allow  the  preparation  to  stand. 
Almost  immediately  the  drop  becomes  turbid  around  the  edges, 
because  of  the  separation  of  minute  crystals  of  silver  chloride; 
these  crystals  increase  slowly  in  size,  but  are  always  very  small, 
requiring  a  moderately  high  power  for  distinguishing  their  form. 
From  ammoniacal  solutions  silver  chloride  seems  to  separate 
almost  invariably  in  the  form  of  cubes  and  hexagonal  and  rec- 
tangular plates.     Only  rarely  are  octahedral  crystals  obtained. 

Of  the  two  recrystallization  methods,  that  with  ammonium 
hydroxide  will  be  found  to  be  the  better,  as  well  as  also  the  more 
convenient,  because  of  the  greater  solubility  of  the  precipitate 
in  this  reagent,  and  because  the  employment  of  ammonium 
hydroxide  eliminates  many  interfering  substances. 

Lead  chloride  is  precipitated  in  the  form  of  white  acicular 
crystals,  irregular  crystallites  and  X-like  dendrites,  soluble  in 
hot  water  and  therefore  easily  removed. 

Mercurous  salts  yield  a  granular  precipitate,  but  sometimes 
minute  needles.  Recrystallized  from  concentrated  hydrochloric 
acid  tetragonal  crystals  may  be  obtained  but  no  cubes  and  part 
of  the  salt  is  converted  into  soluble  mercuric  chloride.  Mercu- 
rous salts  therefore  interfere  with  the  satisfactory  detection  of 
traces  of  silver  by  masking  the  tiny  cubes  of  silver  chloride. 

Thallous  salts  yield  cubes  and  stars. 

Treated  with  ammonium  hydroxide,  silver  chloride  dissolves 
with  the  formation  of  the  compound  AgCl«2NH3  (Isambert). 
If  mercurous  chloride  is  present  the  precipitate  turns  black  under 
the  action  of  the  reagent,  an  insoluble  compound  being  formed 
which  Barfoed  has  shown  to  be  a  mixture  of  metallic  mercury 
and  the  compound  Hg-NH2'C1.  If,  therefore,  silver  chloride 
is  present  only  in  traces  in  a  precipitate  consisting  chiefly  of 
mercurous  chloride,  ammonium  hydroxide  may  dissolve  practi- 
cally no  silver  chloride,  since  the  finely  divided  metallic  mercury 
may  reduce  the  greater  part  of  the  silver  salt  to  metallic  silver. 
(Silver  follows  mercury  in  the  electrochemical  series.)     Under 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       333 

such  conditions  it  is  necessary  to  exercise  the  greatest  care  in 
order  to  avoid  missing  the  little  silver  which  is  present. 

Elements  forming  oxychlorides  may  under  exceptional  con- 
ditions be  precipitated  with  the  silver. 

It  is  also  well  to  bear  in  mind  that  the  addition  of  hydrochloric 
acid  may  force  back  the  dissociation  of  certain  salts  to  a  degree 
causing  the  separation  of  a  solid  phase. 

Precautions. 

When  working  with  concentrated  hydrochloric  acid  or  strong 
ammonia,  great  care  must  be  used  to  avoid  spoiling  the  micro- 
scope and  objectives.     It  is  essential  to  work  rapidly. 

The  drop  is  acidified  with  nitric  acid  because  the  presence  of 
this  reagent  favors  the  agglutination  of  the  particles  of  silver 
chloride,  and  hinders  at  the  same  time  the  precipitation  of  oxy- 
chlorides, etc. 

Decanting  after  precipitation  is  advisable,  since  the  crystal 
form  of  silver  chloride  is  changed  by  many  compounds  when  the 
former  is  crystallized  in  the  presence  of  the  latter.  Still  other 
compounds  completely  ruin  the  test.  Although  there  is,  of 
course,  danger  of  the  occlusion  of  some  of  these  objectionable 
salts  by  the  silver  chloride,  this  difficulty  is  reduced  to  a  mini- 
mum by  avoiding  too  concentrated  test  drops  and  washing  the 
precipitate. 

Washing  the  precipitated  silver  chloride  with  hot  water  re- 
moves the  greater  part  of  the  lead  chloride  which  may  have  been 
carried  down  with  the  silver. 

EXPERIMENTS. 

a.  Precipitate  with  dilute  HC1,  a  test  drop  containing  AgN03.  Separate  and 
wash  the  precipitate;  then  recrystallize  it  by  the  above  described  method,  using 
concentrated  HC1.     Then  repeat  the  experiment,  using  XH4OH  as  the  solvent. 

b.  Make  a  mixture  of  Ag  and  Pb,  test  by  both  recrystallization  methods. 

c.  In  like  manner  test  a  mixture  of  AgN03  and  HgN03. 

d.  Precipitate  with  HC1  a  test  drop  containing  Pb  and  Ag;  recrystallize  the 
precipitate  without  drawing  off  the  solution.  In  like  manner  test  mixtures  of  Ag 
and  Zn,  Ag  and  Cd,  Ag  and  Sb,  Ag  and  Pt,  Ag  and  Sn,  Ag  and  Cu. 

e.  Try  recrystallization  of  AgCl  in  the  presence  of  phosphates,  in  the  presence 
of  sulphates  and  in  the  presence  of  molybdates. 


334 


ELEMENTARY   CHEMICAL   MICROSCOPY 


B.   By  Means  of  Ammonium  Bichromate. 

Acidify  the  test  drop  with  nitric  acid.     Add  a  fragment 
of  the  reagent  at  the  center.     Allow  to  stand  a  few  seconds. 

Dark  red  triclinic  pleochroic  crystals  of  the  formula  Ag2Cro07 
appear  in  the  form  of  thin  plates,  having  a  rectangular  or  more 
or  less  symmetrical  coffin-like  outline.  Aggregates  of  irregular 
broken  scales  are  also  abundant. 

Insufficiently  acidified  drops  or  those  which  are  very  concen- 
trated yield  as  the  first  crop  of  crystals,  tiny  rods  or  needles  so 
dark  colored  as  to  appear  black;  after  a  time  there  will  generally 
separate  in  addition  to  these  rods,  the  characteristic  plates  and 
scales  mentioned  above. 

Cold  solutions  of  lead  yield  only  a  bright  yellow  amorphous 
precipitate.  But  from  hot  solutions,  thin  but  long  and  slender 
monoclinic  prisms  are  formed,  not  however  of  lead  bichromate 
but  having  the  composition  PbCr04.  Lead  chromate  is  soluble 
in  sodium  hydroxide  solutions. 

Mercurous  salts  yield  with  ammonium  bichromate,  in  solutions 
acidified  with  nitric  acid,  a  number  of  different  compounds  (see 
Mercury)  varying  in  composition  and  appearance  according  to 
the  conditions  which  obtain.  There  is,  however,  little  danger  of 
confusing  these  salts  with  the  silver  bichromate,  since  they  all 
appear  as  dark  red  crosses  and  bundles  of  irregular  outline. 
These  compounds  may,  however,  seriously  interfere  with  the 
recognition  of  silver  if  the  latter  is  present  only  in  traces.  Mer- 
curous chromate  is  insoluble  in  sodium  hydroxide,  a  distinction 
from  lead.1 

Silver  bichromate  can  be  recrystallized  from  hot  water,  but 
better  results  follow  the  use  of  dilute  nitric  acid  or  of  ammonium 
hydroxide.  From  hot  nitric  acid  very  beautiful  preparations 
can  be  obtained.  According  to  some  investigators  the  crystals 
which  separate  on  cooling  from  a  hot  neutral  aqueous  solution  of 
the  bichromate  precipitate  are  not  silver  bichromate,  but  normal 
silver  chromate  Ag2Cr04. 

Ammonium  hydroxide  dissolves  silver  bichromate  with  ease. 

1  If,  however,  only  a  minute  quantity  of  sodium  or  potassium  hydroxide  is  used, 
a  red  basic  chromate  of  lead  results. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       335 

The  crystals  separating  from  the  ammoniacal  solution  are,  accord- 
ing to  some  chemists,  complex  salts,  containing  one  or  more  mole- 
cules of  NH3.  The  recrystallized  product  separates  in  the  form 
of  needles,  skeleton  crystals  and  masses  resembling  lichens. 

Unless  the  original  precipitation  was  made  in  nitric  acid  solu- 
tion both  strontium  and  barium  may,  under  unusual  conditions, 
be  precipitated.  It  is  well  to  bear  this  in  mind  when  recrystal- 
lizing  from  ammonia. 

In  the  presence  of  much  lead  the  reaction  often  fails.  Instead 
of  the  dark  red  salt,  small  yellow  prisms  of  entirely  different 
appearance  separate.  In  such  an  event  either  first  remove  the 
lead  with  a  drop  of  dilute  sulphuric  acid  and  then  add  the  bichro- 
mate, or  else  add,  immediately  after  the  fragment  of  the  reagent, 
a  drop  or  two  of  dilute  sulphuric  acid.  Usually  in  a  short  time 
good  crystals  can  be  obtained.  The  use  of  sulphuric  acid  in 
connection  with  the  bichromate  complicates  matters,  since  the 
crystals  separating  in  the  presence  of  the  silver  sulphate  formed 
in  the  reaction  may  be  either  those  of  the  salt  Ag2Cr207  or  the 
salt  Ag2Cr04;  the  latter  compound  is  usually  formed  when  the 
amount  of  nitric  acid  is  small  and  that  of  silver  sulphate  large. 
Normal  silver  chromate  is  isomorphous  with  normal  silver  sul- 
phate, normal  silver  selenate,  and  anhydrous  sodium  sulphate; 
all  are  to  be  referred  to  the  orthorhombic  system.  Because  of 
this  isomorphism  of  the  sulphate  and  chromate  very  interesting 
and  instructive  preparations  may  be  obtained.  Silver  sulphate 
separates  from  solution  generally  in  the  form  of  highly  refrac- 
tive, transparent,  colorless,  rhombic  octahedra,  but  in  the  pres- 
ence of  silver  chromate  these  colorless  octahedra  increase  in  size, 
turn  first  yellow,  and  finally  a  more  or  less  intense  brownish 
red. 

Normal  potassium  chromate  added  to  neutral  solutions  of 
silver  causes  the  precipitation  of  normal  silver  chromate;  but 
when  the  test  drop  is  first  acidified  with  nitric  acid  the  crystals 
separating  probably  consist  of  both  the  chromate  and  bichro- 
mate. When  recrystallized  from  hot  nitric  acid  the  precipitate 
will  usually  consist  of  the  bichromate  alone.  When  ammonium 
hydroxide  is  the  solvent  employed  to  recrystallize  the  silver  chro- 


336  ELEMENTARY   CHEMICAL   MICROSCOPY 

mate,  the  compound  separating  is  thought  to  have  the  formula 

Ag2Cr04  •  4  NH3.1 

Normal  potassium  chromate  produces  in  neutral  or  slightly  acid 
solutions  of  manganous  salts  sheaves  and  bundles  of  a  cinnamon 
brown  manganous  chromate  soluble  in  excess  of  acid.  Bichro- 
mates cause  no  precipitates  in  solutions  of  manganous  salts. 

Precautions. 

The  test  drop  must  be  moderately  concentrated  with  respect 
to  silver. 

When  working  with  test  drops  acidified  with  nitric  acid  there 
is  little  danger  of  any  interference  by  members  of  the  calcium 
group. 

Large  amounts  of  the  salts  of  the  alkalies  seem  to  have  an 
injurious  effect  when  but  little  silver  is  present. 

In  all  analytical  work  it  is  safe  to  assume  that  the  presence  of 
any  elements  which  are  precipitated  as  chromate  or  bichromate 
in  acid  solution  will  interfere  with  the  reaction  for  silver,  particu- 
larly when  such  elements  are  in  excess  of  the  latter. 

White  alloys  believed  to  contain  silver  can  be  tested  for  this 
element  by  drawing  across  them  a  streak  of  a  solution  of  ammo- 
nium bichromate  in  nitric  acid.  The  color  of  the  streak  is  gener- 
ally sufficient  to  indicate  the  presence  or  absence  of  silver,  but 
if  the  streak  of  the  reagent  be  examined  under  the  microscope 
(best  with  an  illuminating  objective  or  some  form  of  vertical 
illuminator)  in  the  presence  of  silver  the  characteristic  dark  red 
crystals  of  silver  bichromate  will  be  easily  distinguished. 

EXPERIMENTS. 

a.  To  a  moderately  concentrated  neutral  test  drop  add  a  fragment  of 
(NH4)2Cr207.     Then  try  K2Cr04. 

b.  Acidify  test  drops  with  HN03,  then  add  the  above  reagents  in  turn. 

c.  Decant  the  mother  liquor  from  a  precipitated  test  drop  and  recrystallize  the 
Ag  salt  by  heating  with  H20.  Try  another  preparation  by  heating  with  dilute 
HNO3.     Recrystallize  a  third  portion  of  the  Ag  compound,  using  NH4OH. 

d.  Make  a  mixture  of  AgN03  and  PbN03,  acidify  with  HXO3,  then  add  a  drop 
or  two  of  dilute  H2S04  and  finally  a  fragment  of  (NH4)2Cr207. 

1  Ladenburg,  Handworterbuch,  10,  713. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       337 

e.  Repeat  the  last  experiment,  adding  this  time  the  (NHj^C^O?  first,  and  then 
the  H2SO4. 

/.  Test  several  different  preparations  containing  mixtures  of  the  Ca  group 
and  Ag. 

g.  Test  a  mixture  of  AgN03  and  HgN03. 

h.  Make  a  rather  concentrated  neutral  test  drop  of  AgN03,  add  a  tiny  crystal 
of  Na2S04.  Study  the  Ag2S04,  which  soon  separates.  Then  add  to  the  prepara- 
tion a  fragment  of  (NH^C^Oy.  Note  well  all  that  takes  place.  If  a  selenate  is 
at  hand,  substitute  it  in  a  new  preparation  for  the  Na2S04. 


C.  By  Means  of  Arsenic  Acid. 

The  reagent  is  made  by  introducing  into  a  drop  of  a 
dilute  solution  of  arsenic  acid  a  tiny  drop  of  dilute  ammonium 
hydroxide;  stir. 

Apply  the  reagent  by  Method  /,  page  251. 

Silver  arsenate  Ag3As04  (hexagonal)  in  the  form  of  a  fine 
granular  precipitate  is  immediately  produced;  later,  thin  plates 
and  plate-like  prisms  appear.  The  majority  of  the  crystals 
which  separate  have  the  appearance  of  hexagonal  plates.  Their 
color  by  transmitted  light  varies  from  a  reddish  yellow  in  very 
thin  plates  to  reddish  brown  with  a  tinge  of  dirty  violet  or  even 
deep  black  as  the  thickness  of  the  crystal  increases. 

Crystallites  bristling  with  long  slender  needles  also  abound. 

Silver  arsenate  is  insoluble  in  acetic  acid,  soluble  in  hot  nitric 
acid  and  easily  soluble  in  ammonium  hydroxide.  Good  prepa- 
rations can  be  obtained  by  recrystallizing  from  either  of  the 
latter  solvents. 

In  case  ammonium  hydroxide  is  employed,  the  colorless  solu- 
tion resulting  contains  the  compound  Ag3As04  •  4  NH3,  as  has 
been  shown  by  Widman.  This  tetra-ammonia  salt  can  be  made 
to  crystallize  in  the  absence  of  air  in  colorless  needles,  but  on 
coming  in  contact  with  the  oxygen  of  the  air  they  turn  red.  It 
follows  from  this  that  the  crystals  obtained  by  recrystallizing 
silver  arsenate  from  ammonium  hydroxide  are  doubtless  of  vari- 
able composition. 

Although  the  crystals  of  silver  arsenate  are  neat,  well  formed 
and  characteristic,  the  reaction  cannot  be  considered  as  a  satis- 
factory one  for  silver  because  of  the  fact  that  most  of  the  other 
metals  usually  associated  with  silver  are  also  precipitated  by 


338  ELEMENTARY   CHEMICAL   MICROSCOPY 

arsenic  acid,  thus  seriously  interfering  with  the  test.  Solution 
of  the  precipitated  arsenate  in  ammonium  hydroxide  and  draw- 
ing off  will  usually  effect  a  partial  separation  at  least,  and  yield 
a  more  satisfactory  test,  but  on  the  other  hand  the  rendering  of 
the  drop  alkaline  may  lead  to  the  separation  of  arsenates  which 
are  soluble  in  acids  but  insoluble  in  alkaline  solution. 

Arsenic  acid  applied  as  indicated  may  yield  with  calcium  salts 
a  separation  of  the  compound  NH4CaAs04  •  6  H20,  ortho- 
rhombic,  isomorphous  with  the  corresponding  phosphate;  the 
crystals  appear  as  large  envelope-like  crystallites  with  more  or 
less  ragged  edges.  If  the  solution  be  dilute  hemimorphic  forms 
identical  with  those  of  ammonium  magnesium  phosphate  are 
seen,  but  generally  of  a  larger  size.  Strontium  yields  minute 
stars  and  crystalline  grains;  barium  a  dense  amorphous  pre- 
cipitate. 

Members  of  the  magnesium  group  yield  colorless  crystalline 
double  ammonium  arsenates  isomorphous  with  their  double 
ammonium  phosphates.  Good  crystalline  compounds  will  be 
obtained  with  the  alkaline  earths  and  with  the  magnesium  group 
only  when  considerable  ammonium  hydroxide  has  been  added 
to  the  reagent  or  when  the  test  drop  is  distinctly  ammoniacal; 
under  these  circumstances  the  detection  of  silver  as  arsenate  may 
be  masked. 

Although  silver  arsenate  is  of  little  value  as  an  identity  test 
for  silver  it  is  of  considerable  use  in  detecting  arsenates. 

Precautions. 

The  arsenic  acid  may  be  added  directly  to  the  test  drop  to 
either  neutral  or  to  weak  nitric  acid  solutions,  but  the  best  and 
most  uniform  results  seem  to  follow  the  procedure  suggested 
above. 

The  amount  of  ammonium  hydroxide  added  to  the  reagent 
drop  must  never  be  sufficient  to  neutralize  all  the  arsenic  acid 
and  give  rise  to  an  alkaline  solution. 

Note. 

It  is  of  theoretical  interest  to  consider  in  connection  with  the 
arsenic  acid  test  for  silver,  the  behavior  of  compounds  of  the 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       339 

elements  analogous  to  arsenic  as  shown  by  their  position  in 
the  Periodic  System.  We  find,  for  example,  crystalline  salts 
of  silver  with  phosphorus,  as  silver  phosphate;  with  antimony, 
silver  antimonate;  with  vanadium,  silver  vanadates;  with  chro- 
mium, silver  chromates;  with  molybdenum,  silver  molybdates. 
Of  these  salts  the  chromates  and  vanadates  can  be  employed  for 
the  detection  of  silver,  but  the  phosphates,  antimonates  and 
molybdates  cannot  be  made  to  yield  sufficiently  characteristic 
results. 

EXPERIMENTS. 

a.  Test  a  neutral  solution  of  AgN03  in  the  manner  suggested  above. 

b.  Recrystallize  a  preparation  of  Ag3As04  from  HN03. 

c.  Try  another  preparation  with  NH4OH. 

d.  Test  a  mixture  of  Ag  and  Pb.     Then  one  of  Ag  and  Hg. 

e.  Try  the  above  reaction  on  salts  of  Ca,  Sr  and  Ba,  first  alone,  then  in  mix- 
tures but  with  no  Ag  present. 

/.   Try  salts  of  Mg,  Zn  and  Cd. 

g.   Try  a  salt  of  Ca  in  the  presence  of  much  NH4C1. 


COPPER. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts  of 
Copper. 

A.  ISOTROPIC.  —  Cuprous    chloride,    bromide    and 

iodide. 

B.  ANISOTROPIC. 

Hexagonal. 

Tetragonal. — Ammonium-copper  chloride;  potas- 
sium-copper chloride. 

Orthorhombic.  -  -  Chloride;   sulphate  plus  4  NH3. 

Monoclinic.  —  Acetate;  potassium-copper  sul- 
phate. 

Triclinic.  —  Sulphate. 

DETECTION. 

A.  By  Means  of  Ammonium  Mercuric  Sulphocyanate. 

The  reagent  is  applied  by  Method  /,  page  251,  to  neutral  or 

weakly  acid  solutions;  it  must  be  neither  alkaline  nor  ammoniacal. 

The  appearance,  properties  and  peculiarities  of  copper  mercuric 


340  ELEMENTARY   CHEMICAL  MICROSCOPY 

sulphocyanate  have  been  discussed  at  length  under  Zinc  on  page 
309,  to  which  the  student  is  referred. 

To  obtain  the  truly  characteristic  moss-like  and  radiating 
crystallites  the  drop  being  tested  must  contain  but  little  copper. 
The  double  sulphocyanate  is  sufficiently  soluble  to  require  several 
minutes  for  its  appearance  in  very  dilute  solution. 

Since  the  zinc  salt  is  much  less  soluble  and  possesses  the  prop- 
erty of  adsorbing  any  copper  present  with  a  change  of  color  from 
white  through  brown  and  black,  a  little  zinc  acetate  or  sulphate 
added  to  the  drop  to  be  tested  before  the  reagent  is  applied  will 
greatly  increase  the  delicacy  of  the  reaction.  Infinitesimal  per- 
centages of  copper  may  be  thus  detected. 

The  sulphocyanate  test  is  the  most  satisfactory  and  generally 
useful  identity  test  for  copper  we  possess. 

EXPERIMENTS. 
These  have  already  been  performed  under  Zinc. 


B.   By  Means  of  the  Triple  Nitrite  Reaction. 

When  copper  alone  is  to  be  tested  for,  proceed  as  follows: 
To  the  moderately  concentrated  drop  add  a  fragment  or  two  of 
sodium  acetate  if  free  mineral  acid  is  present,  if  not  add  a  tiny 
drop  of  dilute  acetic  acid,  next  add  a  fragment  of  lead  acetate 
and  stir  until  dissolved.  Finally  add  a  fragment  of  potassium 
nitrite.  The  black  triple  nitrite  of  potassium,  copper  and  lead 
K2CuPb(N02)6  which  is  formed  has  been  described  under  Lead, 
page  327  (q.v.).  By  adding  rubidium,  cesium  or  thallous  salts 
the  delicacy  of  the  reaction  may  be  greatly  increased. 

If  nickel  is  present  it  will  separate  as  a  triple  nitrite  of  similar 
composition  K2NiPb(N02)6,  light  yellow  or  yellow-brown,  in 
squares  and  cubes  of  larger  size.  They  differ  from  the  copper 
compound  in  never  being  black. 

Cobalt  is  immediately  precipitated  as  insoluble  potassium 
cobalt  nitrite. 

In  testing  alloys  or  mixtures  likely  to  contain  lead,  copper, 
nickel  and  cobalt,  it  is  best  to  modify  the  above  procedure. 
Sodium  acetate  is  first  added,  then  potassium  nitrite  followed  by 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       341 

acetic  acid.  Cobalt  will  immediately  be  precipitated.  If  lead 
and  nickel  or  copper  are  present  the  yellow  or  black  or  both  triple 
nitrites  will  eventually  separate.  If  none  appears,  a  little  lead 
acetate  is  added;  tiny  black  squares  and  cubes  indicate  copper. 
Powerful  oxidizing  agents  must  be  absent. 

EXPERIMENTS. 

a.  Test  for  Cu  in  CuS04;  in  Cu(N03)2. 

b.  Try  the  reaction  in  acid  solution;  in  ammoniacal  solution. 

c.  Try  in  like  manner  a  mixture  of  Cu  and  Ni,  Cu  and  Co. 


C.   Other  Useful  Reactions  for  Copper,  which  may  arise  in 
Testing  for  Other  Elements. 

Cesium  chloride  forms  two  very  characteristic  double 
chlorides  with  copper  CsCl  •  CuCl>  in  golden  yellow  rectangular 
plates,  squares  and  short  stout  prisms  and  a  less  frequently  met 
with  orange  colored  salt  of  unknown  formula.  These  char- 
acteristic double  salts  frequently  appear  when  testing  for  tin, 
antimony,  or  bismuth  with  cesium  chloride  or  on  rare  occasions, 
when  testing  for  aluminum.  Ferric  chloride  also  forms  a  yellow 
double  chloride  with  cesium  chloride.  The  color  and  the  appear- 
ance of  the  cesium  iron  chloride  is  quite  different  from  the  copper 
salt  and  the  combination  does  not  take  place  so  readily. 

Potassium  ferrocyanide  in  acetic  acid  solutions  yields  an 
amorphous  red-brown  precipitate.  Added  to  ammoniacal  solu- 
tions there  appear  after  a  time  white  dendrites  of  copper  ferro- 
cyanide ammonia  2  (NH3)  •  Cu2Fe(CN)6.1  The  addition  of  acetic 
acid  causes  these  dendrites  to  become  red. 


ALUMINUM. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts  of 
Aluminum. 

A.  ISOTROPIC—  The  alums  (I). 

B.  ANISOTROPIC. 

Hexagonal.  —  Sulphate;   chloride  (6  H20). 
Tetragonal. 

1  Behrens,  Anleitung,  p.  75. 


342 


ELEMENTARY   CHEMICAL   MICROSCOPY 

Orthorhombic.  --Nitrate  (usually  M). 
Monoclinic.  —  Nitrate  (or  O) . 
Triclinic. 


DETECTION. 

A .   By  Means  of  Cesium  Sulphate. 

Apply  the  reagent  by  Method  777,  page  252. 

Cesium  alum  CsAl(S04).>  •  12  H20  separates  in  large,  beauti- 
fully formed,  brilliant,  colorless  octahedra,  dodecahedra  or  in 
combinations  of  the  cube  and  octahedron  (isometric).  Dendrites 
and  many  faced  crystal  aggregates  are  also  frequent. 

Test  drops  containing  cesium  alum  have  a  great  tendency  to 
remain  in  a  state  of  supersaturation.  Often  a  single  large  crystal 
only  will  appear.  In  such  an  event,  crushing  the  crystal  and 
drawing  its  fragments  through  the  drop  will  almost  invariably 
yield  a  large  crop  of  well-formed  crystals. 

Schoorl  suggests  keeping  as  a  reagent  a  sample  of  pure  cesium 
alum.  When  testing  for  aluminum  he  adds  cesium  sulphate  (or 
chloride)  and  after  concentration  to  about  the  point  Of  super- 
saturation,  the  tiniest  possible  fragment  of  cesium  alum  is  intro- 
duced into  the  preparation  and  instantly  pressed  upon  and 
crushed  with  a  platinum  wire,  thus  seeding  the  drop  and  causing 
the  immediate  appearance  of  the  alum  crystals,  providing  of 
course  that  aluminum  is  present. 

Testing  for  aluminum  with  cesium  sulphate  leaves  little  to  be 
desired  as  to  accuracy  and  elegance,  but  requires  a  little  practice 
to  learn  just  the  proper  concentration.  Too  dilute  a  test  drop 
requires  very  long  waiting.  Spontaneous  evaporation  leads 
almost  invariably  to  supersaturation.  Evaporation  over  the 
micro-flame  is  very  unsatisfactory.  On  the  other  hand,  the 
addition  of  the  reagent  to  too  concentrated  a  test  drop  gives 
rise  to  the  immediate  formation  of  dendritic  masses  and  skeleton 
crystals.  It  is  true  that  the  experienced  worker  will  usually  at 
once  recognize  these  dendrites  as  due  to  the  presence  of  aluminum, 
but  in  view  of  the  fact  that  beautiful  and  far  more  characteristic 
crystals  can  be  obtained,  the  worker  should  not  be  satisfied  with 
malformed  crystals. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       343 

In  the  presence  of  magnesium  sulphate  there  is  formed  a  double 
sulphate  of  magnesium  and  cesium;  hence  in  dealing  with  such 
cases  it  is  necessary  to  add  a  sufficient  amount  of  cesium  sulphate 
to  permit  of  the  formation  of  both  the  cesium  magnesium  sul- 
phate and  the  cesium  alum.  It  is  very  seldom  that  the  cesium 
magnesium  sulphate  separates;  when  it  does  the  crystals  are  to 
be  referred  to  the  monoclinic  system. 

Manganous  sulphate  will  likewise  form  a  double  sulphate  with 
cesium  sulphate  separating  in  monoclinic  crystals. 

Cesium  alum  is  one  of  a  group  of  double  sulphates  known  as 
"alums,"  having  the  general  formula  M2(S04)3  •  N2S04  •  24  H20, 
where— M--  can  be  Al,  Cr,  Mn,  Fe,  In,  Ga,  Tl;  and  — N- 
Na,  K,  Rb,  Cs,  NH4,  Ag,  or  Tl.  All  alums  are  isomorphous, 
and  are  to  be  referred  to  the  isometric  system.  Theoretically, 
therefore,  one  would  be  led  to  expect  that  the  presence  of  ele- 
ments capable  of  taking  the  place  of  aluminum  in  alums  would  be 
liable  to  interfere  with  the  test  for  aluminum.  But  in  addition  to 
their  property  of  being  able  to  replace  aluminum  in  these  double 
sulphates,  we  must  consider  the  crystallizing  power  of  the  com- 
pounds formed.  It  is  herein  that  lies  the  explanation  of  the 
value  of  cesium  sulphate  over  and  above  that  of  any  other  of  the 
sulphates  we  might  be  inclined  to  select.  Of  the  above  listed 
alum-forming  elements,  aluminum  is  the  only  one  which  unites 
with  cesium  or  rubidium  sulphates  to  form  easily  crystallizable 
alums.  The  other  elements  unite  with  these  two  sulphates  only 
with  difficulty,  and  the  alums  formed  can  be  regarded,  from  a 
microchemical  standpoint,  as  difficultly  crystallizable.  Sodium, 
potassium  and  ammonium  sulphates  readily  unite  to  form  more 
or  less  crystallizable  alums  with  the  other  alum-forming  elements 
as  well  as  with  aluminum. 

Not  infrequently  it  will  be  found  that  cesium  alum  has  a 
marked  tendency  to  adsorb  various  substances  which  may  be 
present,  leading  to  a  modification  of  the  crystal  form  or  to  colored 
solid  solutions. 

Precautions. 

Although  it  is  obvious  that  in  the  case  of  simple  compounds 
converted  into  sulphates  it  is  merely  necessary  to  add  the  reagent 


344  ELEMENTARY   CHEMICAL  MICROSCOPY 

and  allow  the  preparation  to  crystallize,  it  is  essential  that  due 
regard  be  paid  to  (i)  just  the  right  concentration,  (2)  the  absence 
of  much  free  sulphuric  acid,  (3)  the  absence  of  other  free  mineral 
or  organic  acids,  (4)  the  absence  of  colloidal  substances. 

To  avoid  most  of  these  difficulties  it  is  always  advisable  to 
proceed  as  follows:  To  the  drop  to  be  tested  add  ammonium 
hydroxide  in  slight  excess,  decant  the  solution  and  wash  the 
gelatinous  precipitate  with  water.  Then  add  a  drop  of  water 
and  follow  it  with  a  very  little  dilute  sulphuric  acid,  only  just 
enough  to  dissolve  the  aluminum  hydroxide.  Warm  gently; 
cool,  and  to  the  drop  add  a  fragment  of  the  reagent.  After  a  few 
seconds,  beautiful  large  crystals  of  cesium  alum  separate. 

Cesium  chloride  can  be  employed  as  reagent,  providing  that 
the  solution  to  be  tested  contains  a  little  free  sulphuric  acid.  The 
chloride  is,  however,  not  as  satisfactory  as  the  sulphate,  particu- 
larly in  the  hands  of  beginners,  for  cesium  chloride  crystallizes 
in  the  isometric  system,  thus  sometimes  leading  to  confusion. 
Cesium  sulphate,  on  the  contrary,  crystallizes  in  the  ortho- 
rhombic  system.  An  examination  of  a  preparation  containing 
the  latter  salt,  between  crossed  nicols,  will  therefore  permit  of  an 
easy  differentiation  between  crystals  of  cesium  sulphate  and 
those  of  cesium  alum. 

If  cesium  sulphate  is  not  at  hand  it  may  be  prepared  from  the 
chloride  in  this  manner:  Place  a  drop  of  sulphuric  acid  at  the 
corner  of  a  slide  or  on  platinum  foil;  add  a  small  crystal  of 
cesium  chloride  and  evaporate  to  dryness.  If  no  fumes  of  sul- 
phur trioxide  escape,  add  another  drop  of  acid  and  heat  again. 
It  is  evident,  that  by  this  method  of  treatment,  in  the  majority 
of  cases,  it  is  in  reality  primary  cesium  sulphate  that  is  formed, 
and  not  the  normal  sulphate  as  implied  above.  Care  must  there- 
fore be  exercised  in  its  use. 

The  difficulties  often  experienced  with  this  test  by  the  beginner 
are  generally  due  to  too  much  sulphuric  acid  in  dissolving  the 
aluminum  hydroxide  and  to  too  much  acid  in  preparing  the 
cesium  sulphate. 


! 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       345 

EXPERIMENTS. 

a.  To  a  test  drop  consisting  of  a  solution  of  Al2(S04)3  add  a  fragment  of  the 
reagent. 

b.  Precipitate  another  drop  with  NH4OH,  decant,  wash  the  precipitate,  dis- 
solve in  the  least  possible  amount  of  H2SO4  and  test. 

c.  Try  RD2SO4  as  reagent;  then  K2SO4;  Na2S04,  (NH4)2S04.     Try  CsCl. 

d.  Test  for  Al  in  the  presence  of  free  HC1;  free  HNO3. 

e.  Test  preparations  containing  Al  and  Fe;  Al  and  Cr;  Al  and  Mn;  Al,  Fe  and 
Cr;  Al  and  Mg;  Al  in  the  presence  of  phosphates. 

/.   Prepare  slides  of  chrome  alum,  iron  alum,  etc.,  then  mixtures  of  these  various 
alums;  note  isomorphism. 


B.   By  Means  of  Ammonium  Fluoride. 

See  Method  XV,  page  268.    Apply  the  fluoride  in  solid 
form  (Method  777). 

Use  a  celluloid  object  slide. 

From  neutral  solutions  or  those  containing  at  the  most  only  a 
trace  of  free  mineral  acid  a  double  fluoride  separates  having  the 
formula  3  NH4F  •  A1F3  or  considering  this  to  be  an  alumino- 
fluoride  its  formula  may  be  written  (NH4)3A1F6.  It  crystallizes 
in  very  tiny  clear-cut  colorless  octahedra  belonging  to  the  iso- 
metric system. 

Alumino-fluorides  of  the  same  formula  of  potassium,  rubidium, 
cesium  and  sodium  are  known;  they  are  even  less  soluble  than 
that  of  ammonium  and  therefore  can  be  obtained  only  in  such 
minute  crystals  as  to  be  useless  as  a  test.  Lithium  alumino- 
fluoride  is  also  very  insoluble. 

The  ammonium,  potassium,  rubidium  and  cesium  salts  are 
isometric  and  form  isomorphous  mixtures;  but  the  sodium  salt 
is  monoclinic. 

In  these  alkali  fluorine  compounds  the  aluminum  can  be  re- 
placed by  titanium,  chromium,  iron  and  vanadium.  But  in  the 
case  of  zircono-fluorides,  silico-fluorides  (see  page  279)  and 
plumbo-fluorides  the  salts  have  the  composition  M2RF6,  where 
M  is  an  alkali  metal  and  R  may  be  Zr,  Si  or  Pb. 

Crystalline  double  fluorides  of  aluminum  with  copper,  nickel 
and  zinc  have  been  described,  but  these  are  too  soluble  to  appear 
under  the  conditions  which  usually  obtain  in  an  analysis. 


346  ELEMENTARY   CHEMICAL   MICROSCOPY 

Precautions. 

Employ  only  neutral  solutions. 

Always  have  an  excess  of  ammonium  fluoride,  for  if  not  a 
compound  of  different  formula  results  appearing  as  very  tiny 
rods,  worthless  as  an  identity  test  for  aluminum. 

Salts  of  lithium,  sodium  and  iron  must  be  absent. 

The  presence  of  silicon  and  analogous  elements  will  generally 
seriously  complicate  matters,  and  may  ruin  the  test,  owing  to 
the  formation  of  silico-fluorides,  etc.  (See  ammonium  silico- 
fluoride  tests,  under  sodium  and  barium.)  Aluminum  silico- 
fluoride  is  gelatinous,  and  does  not  crystallize. 

Testing  for  aluminum  with  ammonium  fluoride  generally  yields 
results  a  trifle  quicker  than  Method  A,  but  the  delicacy  of  the 
reaction  is  very  little  greater.  Moreover,  Method  B  is  subject 
to  many  complications  and  interferences,  and  there  is  always 
danger,  in  spite  of  great  care,  of  damaging  objectives  by  the 
corrosive  vapors  arising  from  the  test  drop,  since  objectives  of 
moderate  power  and  therefore  short  working  distance  must  be 
employed.  For  these  reasons,  testing  with  ammonium  fluoride 
cannot  be  considered  as  being  as  satisfactory  as  the  cesium 
sulphate  method.  One  of  the  chief  reasons  for  inserting  the  test 
in  this  series  is  the  fact  that  crystals  of  ammonium  alumino- 
fluoride  may  occasionally  appear  when  ammonium  fluoride  is 
being  employed  for  other  purposes,  and  the  presence  of  alu- 
minum is  not  suspected. 

The  method  of  testing  for  aluminum  by  heating  with  ammo- 
nium fluoride  in  a  platinum  cup  has  been  described  under 
Method  XV,  page  270  (q.v.).  The  results  thus  obtained  are  in 
most  cases  somewhat  more  reliable  than  those  given  above  but 
require  more  time,  patience  and  care. 


TIN. 

Crystal  Forms  and  Optical  Properties  of  the  Common  Salts 
of  Tin. 

A.   ISOTROPIC.  --Tetraiodide  (I);  potassium  chloro- 
stannate  (I). 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       347 

B.  ANISOTROPIC. 

Hexagonal. 
Tetragonal. 

Orthorhombic.  —  Tetrabromide. 
Monoclinic.  —  Stannous    chloride  +  2  H20;    stan- 
nous fluoride,  stannic  chlorides. 
Triclinic. 

DETECTION. 

A .   By  Means  of  Cesium  Chloride. 

Apply  reagent  by  Method  /,  page  251. 

In  testing  for  tin  it  is  best  to  evaporate  to  dryness  repeatedly 
with  moderately  concentrated  nitric  acid,  thus  converting  the 
element  into  the  insoluble  dioxide.  The  dry  residue  is  extracted 
repeatedly  with  dilute  nitric  acid  to  remove  interfering  elements 
and  finally  dissolved  in  aqua  regia  and  the  excess  of  acid  removed 
by  evaporation.  Dissolve  the  moist  residue  in  water.  There 
is  thus  obtained  a  compound  which  we  may  term  chlorostannic 
acid,1  with  which  cesium  salts  yield  an  immediate  precipitate 
of  cesium  chlorostannate  Cs2SnCl6  in  the  form  of  tiny  colorless 
highly  refractive  regular  octahedra  and  cubes.  Rubidium  gives 
a  similar  compound  of  greater  solubility  and  therefore  yielding 
larger  crystals,  but  of  sufficiently  high  solubility  to  render  the 
separation  of  the  crystalline  phase  too  slow  to  be  of  practical 
use.  These  three  chlorostannates  are  isomorphous.  The  am- 
monium salt  is  more  soluble  than  the  above  and  the  presence  of 
ammonium  compounds  is  therefore  objectionable;  the  same  is 
true  of  sodium  which  yields  Na2SnCl6  •  5  H20. 

Both  iron  and  copper  are  apt  to  be  adsorbed  by  the  tin  oxide, 
in  such  an  event  yellow  or  red  double  chlorides  of  copper  or  iron 
and  cesium  will  eventually  make  their  appearance.  Occasionally 
if  much  iron  is  present  the  crystals  of  cesium  chlorostannate  are 
colored  yellow. 

Lead  if  present  may  give  octahedra  of  cesium  chloroplumbate 
CssPbCle. 

1  This  compound  may  also  be  regarded  as  a  hydrated  stannic  chloride.  If 
evaporated  to  dryness  there  will  be  obtained  SnCU  •  xH20,  where  x  is  3,  5  or  8. 
All  three  salts  are  crystalline  and  all  can  be  referred  to  the  monoclinic  system. 


348  ELEMENTARY   CHEMICAL   MICROSCOPY 

As  already  noted  antimony  gives  hexagons  and  bismuth  rhombs 
of  the  corresponding  chloroantimonate  and  chlorobismuthate. 

In  the  event  of  no  precipitate  appearing  after  some  time,  add  a 
fragment  of  potassium  iodide.  This  may  lead  to  the  formation 
of  cesium  iodostannate  Cs2SnI6  of  less  solubility  than  the  chloro- 
stannate.  The  iodo-compound  separates  in  yellow  cubes  and 
octahedra.1 

In  the  case  of  simple  salts  or  mixtures  it  is  usually  sufficient  to 
convert  into  chlorides  by  evaporating  with  hydrochloric  acid; 
then  dissolve  in  water,  acidulate  with  hydrochloric  acid  and  add 
the  drop  of  cesium  chloride  solution.  But  in  such  an  event  one 
must  remember  that  double  chlorides  of  Sb,  Bi,  Cu,  Fe,  Al,  Zn, 
Cd,  Pb,  etc.,  will  almost  invariably  separate  if  present. 

If  much  tin  is  thought  to  be  present  use  rubidium  chloride  in 
preference  to  cesium  chloride. 

"Note.  -  -  It  is  of  considerable  theoretical  interest  to  note  that 
in  the  compounds  of  the  type  just  considered  MoRCle,  M2RBr6 
and  M2RI6,  M  may  be  K,  Rb,  Cs,  (NH4)  and  R  may  be  Se,  Te, 
Sb,  Pb,  Sn,  Pt,  Ir,  Os,  Pd,  Ru.  All  salts  of  this  series  are  iso- 
morphous  (Groth). 

EXPERIMENTS. 
Defer  until  Bi  is  being  studied. 


ARSENIC. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts  of 
Arsenic. 

A.  ISOTROPIC.  —  Trioxide  (I,  also,  but  rarely  mono- 

clinic). 

B.  ANISOTROPIC. 

Hexagonal.  —  Triiodide ;  silver  arsenate  (second- 
ary, normal  is  I  ?). 

Tetragonal.  —  Secondary  potassium  arsenate. 

Orthorhombic.  —  Calcium -ammonium  arsenate; 
magnesium-ammonium  arsenate. 

1  It  is  probable  that  the  product  actually  obtained  is  a  solid  solution  of  Cs2Snl6 
in  Cs2SnCl6. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       349 


Monoclinic. —  Primary  ammonium  arsenate;  pri- 
mary sodium  arsenate. 
Triclinic. 

DETECTION. 

A.    Through  the  Formation  of  Arsine  and  its  Reaction  with  a 
Crystal  of  Silver  Nitrate. 

Use  the  distilling  tube,  Fig.  133,  page  248,  as  a  generator, 
as  indicated  in  Figs.  138  and  139. 


~r^, 


J> 


Fig.  139. 


Apparatus  for  the  Detection  of  Arsenic. 

Fit  the  side  tube  with  a  plug  of  soft  wood  P.  Introduce  two  or 
three  fragments  of  arsenic-free  zinc  Z,  and  through  a  pipette 
dilute  hydrochloric  acid  A  (the  acid  will  not  flow  into  the  lower 
part  of  the  tube  until  the  plug  P  is  loosened).  Insert  a  loose 
plug  of  absorbent  cotton  C  which  has  been  soaked  in  lead  acetate 
and  dried.  The  plug  P  is  next  withdrawn.  The  acid  is  allowed 
to  flow  upon  the  pure  zinc;  a  tiny  drop  of  water  5  is  introduced 
into  the  side  tube  and  the  plug  reinserted.  This  drop  makes  a 
tight  seal  and  prevents  loss  of  gas.  The  tube  is  now  tipped 
downward  and  a  tube  drawn  down  to  a  capillary  and  containing 
loosely  a  tiny  crystal  S  of  silver  nitrate  and  one  L  of  lead  acetate 
is  attached  by  means  of  a  short  piece  of  rubber  tube  R.  From 
time  to  time  the  crystal  S  is  examined  to  see  if  it  changes  color. 
If  after  some  minutes  S  remains  clear  and  colorless  remove  P, 
insert  the  material  to  be  tested  by  means  of  a  bit  of  drawn-out 


350  ELEMENTARY   CHEMICAL  MICROSCOPY 

glass  tubing  or  a  fragment  of  solid  may  be  pushed  in  by  means 
of  a  platinum  wire.  Close  the  tube  by  means  of  a  drop  of  water 
and  the  plug  P.  The  reaction  may  be  hastened  by  warming  A 
over  the  micro-flame.  If  arsenic  is  present  the  crystal  of  silver 
nitrate  turns  yellow  due  to  the  formation  of  a  compound  believed 
to  have  the  composition  AsAg3  •  AgN03,  and  rapidly  changes 
to  black  through  the  reduction  to  metallic  silver.  The  lead 
acetate  remains  unchanged  unless  hydrogen  sulphide  is  evolved. 

In  acid  solution  antimony  will  yield  stibine  which  reacts  upon 
silver  nitrate  in  a  similar  manner  although  the  yellow  compound 
is  practically  never  seen.  Phosphine  or  hydrogen  sulphide  turn 
the  silver  nitrate  black  at  once,  but  the  sulphur  compounds  should 
have  been  held  back  by  the  lead  acetate  cotton.  The  crystal  L 
is  introduced  merely  to  make  sure  that  any  blackening  "of  S  can- 
not be  due  to  volatile  sulphur  compounds. 

To  differentiate  between  arsenic  and  antimony  we  may  sub- 
stitute fragments  of  aluminum  for  the  zinc  and  a  solution  of 
potassium  hydroxide  for  the  acid.  Under  these  conditions,  no 
stibine  is  evolved,  only  arsine  passes  off  with  the  hydrogen. 
Metallic  antimony  is  precipitated  in  part  and  deposited  in  part 
upon  the  aluminum. 

In  place  of  a  crystal  fragment  of  silver  nitrate  we  may  employ 
a  fragment  of  mercuric  bromide  or  a  textile  liber  soaked  in  mer- 
curic bromide  and  dried;  in  the  latter  case  a  much  finer  capillary 
tube  can  be  used  and  the  delicacy  of  the  reaction  is  somewhat 
increased.     Arsine  turns  mercuric  bromide  red  or  brown. 

B.   By  Reduction  to  Metallic  A  rsenic  and  Subsequent  Oxidation 
to  Arsenic  Trioxide. 

The  powdered  material  is  mixed  with  a  small  quantity 
of  dry  anhydrous  potassium  ferrocyanide  and  introduced  into  a 
thin  walled  tube  of  hard  glass  drawn  down  to  a  point  and  fused. 
The  tube  is  tapped  gently  to  cause  all  the  material  to  collect  in 
the  tip  of  the  tube.  Heat  the  material  gently  at  first  and  finally 
raise  the  temperature  to  a  red  heat.  The  arsenical  compound  is 
reduced;  arsenic  is  set  free  and  condenses  upon  the  walls  of  the 
tube  as  a  brownish  mirror.  Antimony  will  yield  a  black  or  metal- 
lic mirror;  mercury  a  sublimate  of  tiny  wlvery  spheres.  Certain 
compounds  of  carbon  or  sulphur  may  yield  deposits  upon  the 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       35 1 

glass  closely  resembling  the  arsenic  mirror.  It  is  therefore 
essential  to  carry  the  test  a  step  farther;  to  this  end,  cut  off  the 
closed  tip  of  the  tube  and  heat  the  mirror  over  the  micro-flame. 
The  arsenic  will  be  vaporized  and  oxidized,  collecting  upon  the 
cool  walls  as  Aso03  in  the  form  of  glistening  colorless  highly  re- 
fractive («  =  1-755)  isometric  crystals  in  the  form  of  octahedra 
or  as  derivatives  of  the  octahedron.  These  crystals  are  soluble  in 
potassium  hydroxide  solutions  and  are  precipitated  therefrom 
in  the  form  of  octahedra  by  strong  nitric  acid. 

ARSENATES. 

By  Means  of  Silver  Nitrate. 

Apply  reagent  by  Method  /,  page  251,  to  the  ammoniacal  drop. 

This  reaction  has  already  been  discussed  at  length  under  Silver, 
Method  C,  page  337. 

By  Means  of  Zinc  Chloride  in  Ammoniacal  Solution. 

To  the  test  drop  add  ammonium  hydroxide,  then  apply  the  zinc 
chloride  by  Method  /,  page  251. 

Ammonium  zinc  arsenate  NHjZnAsCX  •  6  H20  separates  in 
the  same  forms  as  those  described  for  ammonium  magnesium 
phosphate  (q.  v.)  with  which  it  is  isomorphous,  as  also  with  the 
compound  NH4ZnP04  •  6  H20 ;  but  the  crystallites  of  the  am- 
monium zinc  arsenate  are  more  feathery  than  those  of  the 
ammonium  magnesium  phosphate.     Phosphates  must  be  absent. 

ARSENITES. 

By  Means  of  Silver  Nitrate. 

Apply  the  reagent  by  Method  /,  page  251,  to  the  ammoniacal 
drop. 

Lemon  yellow  silver  arsenite  is  immediately  precipitated  first 
as  an  amorphous  mass,  later  crystallizing  in  a  variety  of  forms. 
The  first  crystals  appear  as  exceedingly  tiny  acicular  crystals  in 
masses,  stars  and  crosses,  later  as  fusiform  grains,  and  still  later 
as  thin  rods  with  notched  ends,  or  long  irregular  acicular  prisms. 
Eventually  some  oxidation  takes  place  and  there  will  appear 
crystals  of  silver  arsenate.  Silver  arsenite  is  soluble  in  acids 
and  in  ammonium  hydroxide,  hence  the  amorphous  precipitate 
partially  redissolves. 


352 


ELEMENTARY   CHEMICAL  MICROSCOPY 
EXPERIMENTS. 


a.  Test  by  Method  A  the  following:  solutions  of  As203;  of  NaAs02;  of 
H2KAs04;  one  drop  of  commercial  H2S04;  one  drop  of  commercial  HC1;  trying 
first  the  AgN03  crystal  and  then  the  HgBr2  fiber. 

b.  Test  the  above  compounds  by  Method  B. 

c.  Test  the  same  compounds  with  AgN03;  and  finally  with  ZnCl2. 


A. 
B. 


ANTIMONY. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 
of  Antimony. 

ISOTROPIC. 

ANISOTROPIC. 

Hexagonal.  —  Red  tri-iodide ;  strontium-antimonyl 

tartrate;  lead-antimony  1  tartrate. 
Tetragonal.  —  Barium-antimonyl  tartrate  (T  or  O). 
Orthorhombic.  —  Yellow  tri-iodide  (O  or  M) ;  ba- 
rium-antimonyl   tartrate;    potassium-anti- 
monyl  tartrate;  sodium-antimonyl  tartrate. 
Monoclinic.- —  Antimonyl  chloride. 
Triclinic. 

DETECTION. 

A .   By  Means  of  Cesium  Chloride. 

Apply  reagent  by  Method  /,  page  251,  to  the  drop  acidi- 
fied with  hydrochloric  acid. 

A  double  chloride  of  cesium  and  antimony  of  the  formula 
2  CsCl  •  SbCl3  •  2§  H20  separates  in  hexagons  and  elongated  six- 
sided  plates.  Many  of  the  hexagons  show  a  system  of  straight 
or  curving  ribs  extending  from  the  center  to  the  angles  of  the 
hexagons. 

Bismuth  yields  rhombs  or  long  plates  showing  an  hexagonal 
outline,  and  having  a  lower  solubility  than  the  antimony  salt. 

Copper  yields  a  series  of  double  chlorides  varying  in  color  from 
bright  yellow  to  deep  red.  These  salts  usually  separate  in  rec- 
tangular prisms,  but  the  red  compound  sometimes  assumes  forms 
closely  resembling  the  iodo-compounds  referred  to  below. 

Tin  causes  the  immediate  precipitation  of  tiny  regular  octa- 
hedra  of  the  formula  Cs2SnCl6,  a  salt  of  chlorostannic  acid. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       353 

Cesium  chloride  has  remarkable  powers  of  forming  more  or 
less  difficultly  soluble  double  chlorides  with  a  large  number  of 
elements  and  we  may  thus  expect  to  often  find  in  preparations 
to  which  cesium  chloride  has  been  added  an  abundant  crop  of 
well-formed  crystals,  whose  origin  is  puzzling  unless  we  know 
what  elements  are  present. 

The  cesium  chloride  test  is  made  more  satisfactory  and  much 
more  sensitive  by  obtaining  an  iodo-salt  instead  of  that  described 
above.  This  is  accomplished  by  adding  a  fragment  of  potassium 
iodide  to  the  test  drop  after  applying  the  cesium  chloride.  Crys- 
tals of  a  double  iodide  of  cesium  and  antimony  having  the  same 
form  as  the  double  chloride  are  obtained  but  they  are  deep  orange 
yellow  or  orange  red  instead  of  colorless.  The  composition  of 
these  crystals  is  not  well  established,  but  the  weight  of  evidence 
seems  to  be  that  three  molecules  of  Csl  unite  with  two  or  three 
molecules  of  Sbl3,  rather  than  with  Sbl4. 

The  test  thus  performed  is  an  excellent  one,  but  requires  con- 
siderable experience  in  order  to  properly  control  the  conditions. 
The  test  drop  must  be  neither  dilute  nor  concentrated  and  only 
just  sufficient  hydrochloric  acid  should  be  present  to  prevent  an 
antimonyl  compound  from  forming.  It  is  also  better  to  adopt 
for  this  iodide  modification  the  method  of  applying  the  reagents 
suggested  by  Schoorl,1  namely  adding  a  fragment  of  cesium 
chloride  to  one  side  of  the  drop  and  a  fragment  of  potassium 
iodide  to  the  opposite  side. 

The  double  iodide  of  bismuth  separates  in  rhombs  and  elon- 
gated hexagons,  rarely  in  the  regularly  formed  hexagons  of  the 
antimony  salt.  Their  color  is  a  deeper  orange  (or  even  a  red) 
than  that  of  the  antimony  double  iodide. 

Tin  forms  yellow  cesium  iodostannate  in  regular  octahedra. 

Precautions. 

When  iodine  separates  it  is  an  indication  that  too  small  an 
amount  of  potassium  iodide  is  present. 

In  the  event  of  a  precipitate  resulting  upon  the  addition  of 
hydrochloric  acid  at  the  beginning  (Ag,  Pb,  Hg,  Cu)  sufficient 
acid  should  be  added  to  complete  the  reaction.     Decantation  or 

1  Beitrage  2.  mikrochem.  Anal.  Wiesbaden  1909,  p.  49. 


354  ELEMENTARY   CHEMICAL   MICROSCOPY 

filtration  should  then  be  resorted  to  and  the  clear  solution  care- 
fully concentrated  to  remove  the  excess  of  acid  until  a  drop  of 
water  causes  a  precipitate  of  antimonyl  (or  bismuthyl)  chloride. 
Then  very  carefully  add  hydrochloric  acid  with  thorough  stirring, 
until  the  precipitate  just  dissolves. 

EXPERIMENTS. 

Defer  until  Bi  is  being  studied. 

ANTIMONATES. 

The  composition  of  the  various  antimonates  commercially 
available  appears  to  be  quite  uncertain.  The  only  one  of  im- 
portance is  the  potassium  salt  sold  variously  as  potassium  anti- 
monate,  metantimonate  or  pyroantimonate ;  it  usually  conforms 
fairly  closely  to  the  formula  H2K2Sb207  •  6  H20.  It  is  difficultly 
soluble  even  in  boiling  water. 

Sodium  salts  in  neutral  solution  yield,  with  antimonates  of  this 
type,  very  insoluble  sodium  pyroantimonate,  separating  as  tiny 
lenticular  grains  or  larger  fusiform  crystals  singly  or  uniting  in 
more  or  less  globular  masses.  From  dilute  solutions  what  appear 
to  be  tetrahedra,  octahedra  or  rectangular  prisms  are  formed. 
Although  appearing  to  be  isometric  the  crystals  are  to  be  referred 
to  the  tetragonal  system. 

Magnesium  salts  in  neutral  solution  yield  H2MgSb207  •  9  H20 
first  as  an  amorphous  precipitate,  later  crystallizing  in  thin 
transparent  colorless  hexagonal  plates,  and  as  small,  irregular 
spherulites.  Occasionally  stars  or  rosettes  or  short  hexagonal 
prisms  are  obtained.  The  magnesium  salt  is  dimorphic,  being 
either  hexagonal  or  monoclinic  according  to  conditions. 

Of  the  two  tests  that  with  sodium  is  the  more  satisfactory. 

If  it  is  necessary  to  neutralize  a  test  drop  in  testing  for  anti- 
monates use  potassium  carbonate. 

Ammonium  salts  interfere  with  the  sodium  and  magnesium 
tests.  

BISMUTH. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 

of  Bismuth. 
A.   ISOTROPIC. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       355 

B.   ANISOTROPIC. 

Hexagonal.  —  Sulphate  (9  H20). 

Tetragonal.  —  Bismuthyl  chloride. 

Orthorhombic. 

Monoclinic. 

Triclinic.  —  Nitrate  (?) ;   bismuthyl  nitrate. 

DETECTION. 

A.  The  Addition  of  Water  to  neutral  or  very  faintly  acid  so- 
lutions followed  by  the  formation  of  a  heavy  white  amorphous 
or  granular  precipitate  should  lead  to  the  suspicion  of  the 
presence  of  bismuth.  From  the  chloride,  the  compound  BiOCl 
is  obtained  and  from  the  nitrate  BiON03  •  H20. 

B.  By  Means  of  Potassium  Sulphate. 

This  test  has  been  discussed  at  length  under  Sodium, 
Method  B,  page  276,  and  also  under  Potassium,  Method  B, 
page  284,  to  which  the  student  is  referred  for  details. 

Neither  arsenic,  antimony,  nor  tin  yield  a  crystalline  deposit. 
The  test  is  therefore  one  of  the  most  satisfactory  for  the  recog- 
nition of  bismuth,  providing  lead  is  absent.  Lead  yields  a  gran- 
ular or  amorphous  (or  rarely  crystalline)  precipitate  with  potas- 
sium sulphate.  It  is  therefore  necessary  to  first  remove  the 
lead  by  precipitating  with  sulphuric  acid  in  the  presence  of 
nitric  acid  before  proceeding  to  test  for  bismuth.  With  this 
end  in  view  add  to  the  solution  to  be  tested  nitric  acid,  then  a 
drop  of  very  dilute  sulphuric  acid  —  if  no  precipitate  results, 
evaporate  until  fumes  of  sulphur  trioxide  are  formed.  Then 
proceed  as  described  under  Method  77,  page  252,  Experiment  a. 
If  a  precipitate  forms  with  the  sulphuric  acid  decant,  centrifuge 
or  filter  the  solution  to  remove  the  lead,  after  which  evaporate 
with  sulphuric  acid  and  proceed  as  above. 

C.  By  Means  of  Cesium  Chloride. 

This  test  has  already  been  discussed  under  Antimony, 
Method  A,  page  352. 

The  only  specific  difference  between  the  double  chlorides  of 
these  two  elements  is  that  with  bismuth  there  is  a  greater  ten- 
dency toward  rhombic  plates.  Conversion  into  double  iodides 
gives  a  salt  darker  colored  than  that  with  antimony. 


356  ELEMENTARY   CHEMICAL  MICROSCOPY 

A  great  excess  of  hydrochloric  acid  seriously  reduces  the 
delicacy  of  the  reaction,  while  nitric  and  sulphuric  usually  pre- 
vent the  separation  of  typical  crystals. 

The  student  must  bear  in  mind  the  caution  given  under  anti- 
mony that  cesium  chloride  has  a  strong  tendency  to  form  double 
salts,  especially  with  lead,  copper,  cadmium,  zinc,  aluminum,  etc. 

EXPERIMENTS. 

a.  Try  CsCl  upon  a  solution  of  Sn  in  HC1. 

b.  Try  the  test  upon  Sb  in  HC1  solution;  upon  Bi  in  HC1  solution. 

c.  Try  converting  the  chloro-salts  of  these  three  elements  into  the  iodo  com- 
pounds. 

d.  Try  testing  for  Sb  and  Bi  in  turn  in  the  presence  of  a  little  Cu. 

e.  Try  mixtures  in  which  some  of  the  other  metals  are  present  which  form 
crystallizable  double  chlorides  with  CsCl. 


D.   Other  Important  Tests. 

With    Primary    Potassium    Oxalate.     (See    Manganese, 
Method  A,  page  359.) 

With  Ammonium  Bichromate.     (See  Silver,  Method  B, 
page  334.) 

CHROMIUM. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 
of  Chromium. 

A.  ISOTROPIC.     Chrome  alums  (I). 

B.  ANISOTROPIC. 

Hexagonal. 

Tetragonal. 

Orthorhombic.--Ba.Ymm  chromate  (or  M) ;  calcium 
chromate  (or  M);  potassium  chromate; 
silver  chromate;  sodium  chromate;  stron- 
tium chromate  (or  M) ;  zinc  chromate.1 

Monoclinic.  —  Ammonium  bichromate;  ammonium 
chromate;  barium  chromate  (or  O) ;  calcium 
chromate  (or  O);  lead  chromate;  strontium 
chromate  (or  O). 

Triclinic.  --Potassium  bichromate;  silver  bichro- 
mate;2 sodium  bichromate. 

1  In  the  presence  of  FeS04  •  7  H20  the  salt  separates  monoclinic. 

2  Ag2Cr207  dissolved  in  water  decomposes  into  Ag2Cr04  and  Cr03. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       357 

DETECTION. 

A.  In  simple  salts  we  may  obtain  the  following  colors  and 
reactions: 

a.  Soluble  chromates  are  yellow,  bichromates  red,  their  solutions 
yellow.  Solutions  of  chromium  salts  where  chromium  acts  as  a 
base,  when  heated  in  acid  solution,  are  green. 

b.  Chromium  yields  with  ammonium  hydroxide  a  bluish  or 
greyish  green  or  greyish  lavender  hydroxide.  In  the  presence 
of  ammonium  salts,  especially  ammonium  chloride,  this  hydrox- 
ide is  partially  soluble  with  the  formation  of  the  compound 
CrCl3  •  4  NH3.  Boiling  drives  off  the  ammonia  and  chromium 
is  completely  precipitated  as  Cr(OH)3. 

c.  Silver  nitrate  gives  in  solutions  weakly  acid  with  nitric  acid 
a  characteristic  deep  red  chromate  with  both  chromates  and 
bichromates  (see  Silver,  Method  B,  page  334).  In  neutral 
solutions  the  bichromate  yields  a  crystalline  silver  chromate 
somewhat  more  readily  than  the  bichromate  but  the  difference 
is  too  slight  to  be  of  any  practical  use  in  differentiating  between 
the  salts. 

d.  Alkali  chromates  added  to  neutral  solutions  of  manganous 
salts  give  a  characteristic  manganous  chromate,  but  alkali  bi- 
chromates give  no  such  reaction  (see  Manganese,  Method  B, 
page  360) . 

B.  By  Conversion  into  Cesium  Chrome  Alum. 

To  a  drop  of  the  solution  to  be  tested  add  ammonium 
hydroxide.  Should  a  reddish  liquid  result,  boil.  Decant  the 
solution  from  the  bluish  or  greenish  precipitate.  Wash  the  pre- 
cipitate once  or  twice.  Add  a  tiny  drop  of  water  and  then  very 
carefully  the  least  possible  amount  of  dilute  sulphuric  acid  which 
will  just  dissolve  the  precipitate.  Evaporate  carefully  nearly  to 
dryness  and  add  a  tiny  drop  of  water.  Finally  introduce  near 
the  center  of  the  drop  a  fragment  of  cesium  sulphate.  Cesium 
chrome  alum  will  almost  immediately  separate  in  characteristic 
alum  crystals,  the  octahedron  and  dodecahedron  predominating 
(isometric).  These  crystals  have  a  faint  bluish  tint  by  trans- 
mitted light.  The  peculiar  purple  color  of  chrome  alum  will 
not  be  seen  unless  they  attain  a  relatively  large  size  and  reflec- 


358  ELEMENTARY   CHEMICAL   MICROSCOPY 

tions  from  their  faces  become  noticeable.  To  be  of  value  as  a 
test  for  chromium  both  the  crystal  form  and  color  must  be  taken 
into  account. 

Free  mineral  acids  should  be  absent,  as  also  the  salts  of  organic 
acids. 

In  general  we  must  observe  the  same  precautions  as  in  test- 
ing for  aluminum  with  cesium  sulphate.  (See  Aluminum,  Method 
A,  page  342.) 

It  is  obvious  that  other  "alum"  forming  elements,  such  as 
aluminum,  iron  and  manganese,  must  be  absent  or  present  only 
in  traces. 

Since  all  the  alums  are  isomorphous  it  is  often  possible  to  start 
crystallization  by  introducing  into  the  test  drop  an  infinitesimal 
trace  of  potash  alum  when  by  chance  the  preparation  shows  a 
tendency  to  supersaturate  and  no  crystals  form,  or  even  better, 
add  a  similar  tiny  fragment  of  cesium  alum.  In  such  an  event 
we  must  place  our  chief  dependence  upon  the  color  of  the  crystals 
separating. 

EXPERIMENTS. 

Test  simple  sails  of  Cr  as  described  above,  then  employ  more  or  less  complex 
mixtures. 


C.   Detection   of  Chromium   in   Complex  Mixtures   such   as 
Alloys,  etc. 

Method  of  Behrens.1  -  -  Place  the  finely-divided  material  on 
an  object  slide.  Add  a  fair-sized  drop  of  concentrated  nitric 
acid,  heat  to  boiling,  decant  the  acid  to  another  slide  and  treat 
the  residue  again  in  the  same  manner.  Repeat  until  all  is  dis- 
solved or  until  sufficient  material  has  passed  into  solution. 
Unite  all  the  drops  and  evaporate  to  dryness.  By  means  of  a 
tiny  spatula  carefully  scrape  off  the  dry  mass  into  a  platinum 
cup  or  upon  a  piece  of  platinum  foil.  Add  a  very  small  quantity 
of  sodium  carbonate-potassium  nitrate  fusing  mixture  (3:1)  and 
heat  until  a  clear  fusion  results,  adding  more  fusing  mixture  if 
necessary,  but  being  careful  to  use  no  more  than  absolutely  neces- 
sary.    The  yellow  fused  mass  is  dissolved  in  water,  concentrated 

1  Behrens,  Anleitung,  1  Auf.  186. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       359 

to  small  bulk,  acidified  with  acetic  acid,  a  trace  of  sulphuric 
acid  added  and  into  the  drop,  a  drop  of  silver  nitrate  is  caused 
to  flow.  Silver  sulphate  will  first  separate  in  its  characteristic 
form  but  will  be  colored  yellow  or  red  through  the  solid  solution 
of  silver  chromate  in  it.  Later  the  red-brown  or  blackish  crys- 
tals of  silver  chromate  appear. 

EXPERIMENTS. 

a.  Look  over  notebook  records  of  experiments  made  under  Silver  —  Exps., 
Method  B,  page  336.  Similar  crystals  will  be  obtained  upon  testing  for  Cr  with 
AgN03. 

b.  Test  for  Cr  in  several  different  Cr  compounds  by  Method  B. 

c.  Test  by  Method  B  in  Cr  salts,  mixed  with  Al,  Fe,  Cu,  Ni. 

d.  Test  for  Cr  in  chrome  iron. 


MANGANESE. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 
of  Manganese. 

A.  ISOTROPIC. 

B.  ANISOTROPIC. 

Hexagonal. 

Tetragonal. 

Orthorhombic.  —  Potassium  permanganate. 

Monoclinic.  —  Acetate  (ous) ;  chloride  (ous) ;  am- 
monium-manganous   sulphate;    potassium- 
manganous    sulphate ;    sodium-manganous 
sulphate. 
Triclinic.  —  Sulphate  (ous). 

DETECTION. 

A.   With  Manganous  Salts  Oxalic  Acid  or  Primary  Potassium 
Oxalate  forms  Characteristic  Crystals  of  Manganous  Oxalate. 

Obtain  a  thin  uniform  film  of  dry  potassium  oxalate  upon 
the  slide;  Method  IV,  page  255.  Draw  across  this  film  the 
neutral  solution  of  the  material  to  be  tested  or  a  solution  slightly 
acidified  with  acetic  acid.  Six-armed  stars  of  MnC2Qi  •  3  HjO 
separate.  These  stars  result  from  the  intersection  of  thin  twinned 
prisms.  They  polarize  strongly,  extinguish  parallel  to  their 
length  and  exhibit  brilliant  polarization  colors. 


360  ELEMENTARY    CHEMICAL   MICROSCOPY 

This  test  is  excellent  when  pure  manganous  salts  are  being 
dealt  with,  but  is  seriously  affected  by  much  alkali  and  ammonium 
salts  or  by  the  presence  of  those  elements  readily  precipitated  as 
oxalate,  for  example,  the  elements  of  Group  VIII  of  the  Periodic 
System,  or  those  of  Group  II. 

Free  mineral  acids  seriously  interfere. 

With  solutions  highly  concentrated  with  respect  to  manganese 
no  reaction  will  be  obtained  nor  will  satisfactory  results  follow 
the  use  of  too  dilute  test  drops. 

Silver,  lead,  mercurous  and  stannous  salts  should  be  absent. 

EXPERIMENTS. 

a.  Test  as  above  MnS04.  Then  try  the  addition  of  a  drop  of  H2C2O4  to  a  test 
drop  by  Method  7,  page  251. 

b.  Try  effects  of  free  acids  upon  the  test. 

c.  Test  mixtures  of  MnS04  with  members  of  Group  VIII. 


B.   By  Means  of  Potassium  Chr  ornate. 

Apply  reagent  to  test  drop  by  Method  7/7,  page  252. 

Sheaves  of  yellowish  brown,  acicular,  strongly  pleochroic 
crystals  separate  from  neutral  or  feebly  acid  solutions;  but  from 
drops  containing  a  trace  of  free  nitric  acid  stout  dendritic  masses 
and  clusters  of  yellowish  brown  prisms  are  obtained.  The  test 
drop  should  be  moderately  concentrated. 

Nitric  acid  greatly  slows  down  the  reaction  and  if  present  in 
more  than  traces  prevents  the  formation  of  crystals.  The  other 
mineral  acids  behave  in  a  similar  fashion. 

With  pure  manganous  salts  this  test  is  excellent,  but  is  of 
little  value  in  the  presence  of  silver,  lead,  mercury  or  in  fact  any 
element  forming  a  difficultly  soluble  chromate. 

See  Silver,  Method  B,  page  334;  Mercury,  Method  B,  page 
321. 

Potassium  bichromate  applied  as  above  gives  no  crystalline 
precipitate. 

EXPERIMENTS. 

a.  Test  a  drop  of  MnS04  with  K2Cr04;  with  K2Cr207. 

b.  Repeat  the  test,  previously  acidifying  with  HN03;  with  HC1;  with  HC2H302. 

c.  Repeat  in  the  presence  of  Ag,  of  Pb. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       361 

C.    Through  Fusion  with  a  Mixture  of  Sodium  Carbonate  and 
Potassium  Nitrate. 

The  fusion  should  be  made  in  a  small  platinum  cup  or 
upon  platinum  foil,  using  the  smallest  possible  amount  of  the 
fusing  mixture  which  will  react  with  the  unknown.  It  is  always 
wise  to  first  obtain  the  hydroxide  or  oxide  and  employ  this 
material  for  the  fusion. 

If  manganese  is  present  a  green  color  is  obtained,  due  to  the 
formation  of  manganates  of  sodium  and  potassium  Na2Mn04, 
K2Mn04. 

Iron  and  chromium  mask  the  reaction. 

EXPERIMENTS. 

a.  Test  several  different  Mn  compounds  by  fusing  on  platinum  foil  or  in  a  bead 
on  Pt  wire. 


D.   By  Means  of  Phosphates  in  Ammoniacal  Solution. 

Manganous  salts  are  precipitated  as  NH4MnP04  •  6  H20. 
See  Magnesium,  Method  B,  page  304;  Nickel,  Method  B, 
page  365;   Cobalt,  Method  C,  page  367. 

The  hemimorphic  crystals  obtained  usually  grow  somewhat 
longer  than  those  of  magnesium  but  are  otherwise  identical. 
They  are  proved  to  be  due  to  manganese  by  adding  hydrogen 
peroxide  which  causes  them  to  turn  brown. 


E.   By  Means  of  Sodium  Bismuthate. 

Dissolve  the  material  in  concentrated  nitric  acid  and 
evaporate  the  solution  to  dryness.  Dissolve  in  dilute  nitric  acid, 
add  several  small  portions  of  sodium  bismuthate,  stirring  after 
each  addition,  allow  to  stand  a  short  time;  a  pink  or  purple  color 
results  with  a  precipitation  of  brown  oxide  of  manganese.  Next 
add  very  carefully  in  tiny  fragments  just  sufficient,  no  more, 
sodium  thiosulphate  to  dissolve  the  precipitated  oxide.  A 
colorless  milky  drop  results;  add  a  drop  of  nitric  acid  (1  :  4) 
and  stir  thoroughly.  Now  again  add  carefully  and  slowly  a 
very  little  at  a  time  sodium  bismuthate.  A  beautiful  pink  or 
purple  color  is  developed  due  to  the  permanganate  formed. 


362  ELEMENTARY    CHEMICAL   MICROSCOPY 

To  complete  the  test  add  a  fragment  of  rubidium  chloride, 
stir,  add  a  drop  of  water  and  allow  a  drop  of  perchloric  acid  to 
flow  into  the  drop.  Crystals  of  rubidium  perchlorate  are  imme- 
diately formed,  taking  up  the  permanganate  in  solid  solution  and 
yielding  pink  or  purple  crystals. 

EXPERIMENTS. 

Test  this  method  first  upon  pure  Mn  salts,  then  upon  mixtures  of  other  elements 
with  Mn. 


IRON. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 
of  Iron. 

A.  ISOTROPIC.     Iron  alums  (I). 

B.  ANISOTROPIC. 

Hexagonal.  — -  Chloride  (when  sublimed) . 

Tetragonal. 

Orthorhombic. — Ammonium-ferric  chloride;  oxa- 
late (ous). 

Monoclinic.  --  Sulphate  (ous);1  ammonium -ferrous 
sulphate;  sodium-ferric  oxalate;  potassium- 
ferric  oxalate. 

Triclinic. 

DETECTION. 

A.   By  Means  of  Potassium  Ferrocyanide. 

To  the  test  drop,  apply  a  fragment  of   the  reagent  by 
Method  777,  page  252. 

A  dark  blue  precipitate  or  color  indicates  iron.  The  precip- 
itate is  soluble  in  alkalies,  insoluble  in  acids.  It  is  therefore 
always  best  to  acidify  with  hydrochloric  acid  before  adding  the 
ferrocyanide. 

The  presence  of  much  copper  may  seriously  interfere  with  the 
test  because  of  the  formation  of  brown  copper  ferrocyanide. 

1  But  if  magnesium  sulphate  is  present,  orthorhombic. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       363 

EXPERIMENTS. 

a.  Test  for  Fe  in  simple  salts. 

b.  Test  in  complex  mixtures  with  other  elements  which  will  be  precipitated  by 

K4Fe(CN)6. 

NICKEL. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts 
of  Nickel. 

A.  ISOTROPIC. 

Ammonia  nickel  nitrate  (I). 

B.  ANISOTROPIC. 

Hexagonal. 

Tetragonal. 

Orthorhombic.  —  Sulphate. 

Monoclinic.  — Acetate;  chloride;  nitrate;  sulphate; 
ammonium-nickel  sulphate;  potassium- 
nickel  sulphate. 

Triclinic. 


DETECTION. 

A .   By  Means  of  Dimethyl  Glyoxime, 


CH3  -  C  =  NOH 


CH3  -  C  =  NOH 


To  a  drop  of  the  solution  to  be  tested  add  ammonium  hydroxide 
until  in  slight  excess.  Decant  the  solution  of  the  hydroxides 
which  have  been  dissolved  by  the  ammonium  hydroxide,  from 
those  which  are  insoluble.  Close  to  the  clear  ammoniacal  drop 
place  a  large  drop  of  a  freshly  prepared  saturated  solution  of 
dimethyl  glyoxime.  Cause  the  ammoniacal  drop  to  flow  into 
the  reagent. 

Nickel  yields  an  immediate  rose-pink  or  magenta-colored 
precipitate  —  at  first  amorphous  in  character,  later  changing  into 
a  felt  of  exceedingly  fine  acicular  crystals.  Near  the  edges  of 
the  crystalline  mass  tiny  needles  form  in  star-like  and  irregular 
bristling  clusters.  Often  a  yellow  precipitate  is  first  formed, 
changing  only  slowly  into  pink. 

The    nickel    salt    of    dimethyl    glyoxime   has    the    formula 


364  ELEMENTARY    CHEMICAL   MICROSCOPY 

Ni(C4H7N2Oo)2.      No  other  element  yields  a  similar  appearing 
compound. 

The  reaction  is  an  exceptionally  sensitive  one;  exceedingly 
small  amounts  of  nickel  may  be  thus  detected  save  in  the  pres- 
ence of  large  amounts  of  cobalt  or  copper.  Neither  cobalt *  nor 
copper  alone  yield  a  precipitate,  but  both  these  metals  mask  or 
prevent  the  formation  of  the  typical  nickel  compound;  a  yellow 
amorphous  precipitate  results  in  which  can  be  found  only  a  few 
masses  of  the  pink  needles. 

Copper  can  be  easily  removed  by  deposition  upon  a  piece  of 
zinc  foil  prior  to  the  addition  of  the  ammonium  hydroxide. 
This  is  accomplished  by  placing  the  weakly  acid  drop  upon  a 
clean  bright  piece  of  zinc.  As  soon  as  a  black  spot  is  formed 
the  drop  is  decanted  to  a  new  position,  and  as  soon  as  it  is  ob- 
served that  the  zinc  is  not  at  once  stained  the  drop  is  decanted 
upon  an  object  slide,  ammonium  hydroxide  added  and  the  test 
for  nickel  applied. 

Cobalt  may  be  removed  by  adding  to  the  almost  neutral  drop, 
a  fragment  or  two  of  potassium  nitrite,  warming  to  hasten  solu- 
tion, and  then  adding  a  drop  of  acetic  acid.  Potassium  cobalt 
nitrite  is  precipitated.  After  a  few  seconds  the  liquid  is  de- 
canted from  the  precipitate  which  clings  tenaciously  to  the  glass 
and  ammonium  hydroxide  is  added,  ignoring  any  few  tiny  par- 
ticles of  the  nitrite  which  may  have  been  carried  over.  The 
glyoxime  test  can  now  be  applied  with  assurance  of  detecting 
nickel  if  present. 

An  excess  of  neither  silver  nor  zinc  appears  to  influence  the 
reaction  for  nickel. 

Dimethyl  glyoxime  gives  with  iron  salts  a  red  color.  In 
testing  for  nickel,  therefore,  we  often  obtain  an  indication  of 
the  presence  of  iron  in  spite  of  the  fact  that  ammonium  hydroxide 
has  been  added;  for  in  the  presence  of  ammonium  salts  the 
addition  of  ammonium  hydroxide  to  ferrous  solutions  will  not 
precipitate  all  the  iron,  owing  to  the  formation  of  soluble  double 

1  All  the  samples  of  cobalt  salts  sold  as  C.P.  tested  by  the  author  have  given  a 
slight  precipitate  with  the  reagent,  probably  due  to  traces  of  nickel  present  in  the 
material. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       365 

salts,  such  as  (NH4)2S04  •  FeS04  or  2  NH4C1  •  FeCL>.  Non- 
volatile organic  acids  prevent  the  precipitation  of  ferric  hydroxide 
and  the  ferric  salts  thus  remaining  in  solution  will  react  with 
the  glyoxime. 

B.   Other  Tests  for  Nickel. 

1 .  Triple  nitrite  of  lead  nickel  and  potassium  K2PbNi(N02)6. 
See  Lead,  Method  C,  page  327;  Copper,  Method  B,  page  340. 

2.  Ammonium  nickelous  phosphate  NH4NiP04  •  6  H20. 
See  Magnesium,  Method  B,  page  304.  This  salt  is  isomorphous 
with  the  magnesium  salt. 

Note.  —  The  addition  of  hydrogen  peroxide  causes  no  change 
in  the  color  of  the  crystals  of  ammonium  nickel  phosphate,  but 
will  turn  those  of  cobalt  brown. 

EXPERIMENTS. 

a.  Try  the  glyoxime  reaction  on  salts  of  Ni  in  NH4OH  and  in  acid  solution;  and 
in  different  concentrations. 

b.  Try  test  upon  Co  compounds. 

c.  Make  a  mixture  of  Ni  and  Co  and  test. 

d.  Test  for  Ni  in  the  presence  of  much  Cu. 

Remove  the  Cu  from  a  drop  by  means  of  metallic  Zn  and  test  again.  Then  try 
the  detection  of  Ni  in  the  presence  of  much  Fe. 

e.  Apply  the  phosphate  test  to  a  Ni  salt  and  as  soon  as  the  crystals  are  well 
formed,  allow  a  drop  of  H202  to  flow  into  the  drop.  Repeat  the  process  with  a 
Co  salt. 


COBALT. 

Crystal  Forms  and  Optical  Properties  of  Common  Salts  of 
Cobalt. 

A.  ISOTROPIC. 

B.  ANISOTROPIC. 

Hexagonal. 
Tetragonal. 
Orthorhombic.  —  Ammonium-cobalt     phosphate; 

purpureo-chloride  (pseudotetragonal) . 
Monoclinic.  -  -  Acetate ;    chloride ;    luteo-chloride ; 

nitrate;  potassium-cobalt  sulphate;  roseo- 

chloride;   sulphate. 
Triclinic. 


366  ELEMENTARY   CHEMICAL   MICROSCOPY 

DETECTION. 

A.   By  Means  of  Ammonium  Mercuric  Sulphocyanate. 

See  Zinc,  Method  A,  page 307;  Copper,  Method  A,  page 

339- 

Mercury  cobalt  sulphocyanate  Hg(CNS)2  •  Co(CNS)2  sepa- 
rates as  dark  blue  prisms,  usually  in  irregular  clusters.  Its 
solutions  have  the  tendency  to  supersaturate  and  it  is  therefore 
necessary  to  give  the  reaction  considerable  time,  or  even  evapo- 
ration over  the  micro-flame  may  be  advisable.  Crushing  the 
first  crystals  appearing  near  the  circumference  of  the  drop  and 
drawing  the  fragments  across  often  expedites  the  reaction. 

Nickel  yields  no  crystals  and  does  not  interfere  unless  in  ex- 
cessively great  amount. 

Precautions. 

The  test  drop  should  be  neutral  or  only  slightly  acid  with 
acetic  acid,  but  must  not  be  alkaline. 

Better  results  are  to  be  obtained  with  mineral  acid  salts  than 
with  those  of  organic  acids. 

EXPERIMENTS. 

The  student  should  refer  to  his  notes  under  Zn,  where  the  results  of  his  ex- 
perience with  the  reagent  upon  Co  should  be  found. 


B.  By  Means  of  Potassium  Nitrite. 

To  the  neutral  or  slightly  acid  drop  add  a  fragment  of 
potassium  nitrite.  Stir.  Then  warm  and  add  a  drop  of  acetic 
acid. 

Potassium  cobalt  nitrite  3  KNO2  •  Co(N02)3  •  i|  H20  is  im- 
mediately precipitated  in  the  form  of  tiny  cubes,  so  minute  as 
to  simulate  an  amorphous  or  finely-granular  deposit.  These 
crystals  appear  black  by  transmitted  light,  yellow  by  reflected 
light.  From  hot  solutions  there  may  sometimes  be  obtained 
crystals  recognizable  as  cubes  and  octahedra. 

This  test  has  its  greatest  value  in  a  negative  way  since  failure 
to  obtain  the  very  insoluble  double  nitrite  may  be  considered  as 
indicative  of  the  absence  of  cobalt. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ELEMENTS       367 

Upon  obtaining  a  yellow  precipitate,  decant  the  supernatant 
liquid,  convert  the  double  nitrite  into  the  chloride,  nitrate  or 
sulphate  and  test  for  cobalt  by  Method  A . 

EXPERIMENTS. 

These  have  already  been  tried  under  Lead,  Method  C,  page  329  (q.v.). 


C.  Other  Tests  for  Cobalt. 

As  ammonium  cobaltous  phosphate  NH4C0PO4  •  6  H20 ; 
isomorphous  with  the  magnesium,  nickel  and  manganese  am- 
monium phosphates.     See  Magnesium,  Method  B,  page  304. 

Add  hydrogen  peroxide  and  warm.  The  cobalt  compound 
turns  brown. 

THE    QUALITATIVE   ANALYSIS    OF   MATERIAL   OF   UNKNOWN 
BUT   OF   SIMPLE    COMPOSITION. 

The  following  brief  outline  may  serve  as  a  guide  to  the  steps 
to  be  taken  in  the  microchemical  analysis  of  the  simple  "un- 
knowns" which  will  be  given  to  the  student  for  practice. 

1.  Examine  the  material  with  a  low  power. 

2.  If  found  to  consist  of  several  components,  try  to  isolate 
them,  using  forceps,  or  scraping  off  particles  with  a  knife  point, 
a  file  or  a  tiny  drill. 

3.  Test  solubilities  in  water,  HN03,  and  NH4OH  and  note 
whether  the  solutions  obtained  yield  crystals  on  evaporation. 

4.  Subject  the  crystalline  material  to  polarized  light  and  deter- 
mine whether  it  is  isotropic  or  anisotropic.  If  the  latter,  whether 
it  exhibits  parallel  or  oblique  extinction.  This  should  afford 
a  clue  as  to  the  probable  nature  of  the  salts  obtained. 

5.  Add  a  drop  of  dilute  HC1 -- presence  or  absence  of  Pb, 
Ag,  Hg,  (Cu).  Decant,  and  test  solution  as  in  6.  To  residue 
add  NH4OH. 

6.  Add  a  drop  of  dilute  H2S04  --  presence  or  absence  of  Ca, 
Sr,  Ba,  Pb,  Ag,  Hg,  (Sb,  Bi),  and  salts  of  low  solubility. 

7.  Add  to  a  drop,  a  drop  of  ammonium  mercuric  sulpho- 
cyanate  —  presence  or  absence  of  Zn,  Cu,  Cd,  Co,  Fe,  (Ag),  (Pb), 
(Au),  (Mn). 


368  ELEMENTARY   CHEMICAL  MICROSCOPY 

8.  Add  to  a  drop  a  little  CsCl  and  KI  —  presence  or  absence 
of  Pb,  Sn,  Hg,  Ag,  Sb,  Bi,  (Cu). 

9.  Add  to  HC1  solution  H-PtCle  —  presence  or  absence  of  K, 
NK,,  Na. 

10.  Add  zinc-sulphide-fiber,  then  at  once  a  drop  of  NH4OH 
-  presence  or  absence  of  Pb,  Ag,  Hg,  Cu,  Bi,  Cd,  As,  Sb,  Sn, 
Ni,  Co,  Fe,  Al,  Cr,  Mn,  Au,  and  salts  insoluble  in  NH4OH. 

n.  Or  instead  of  proceeding  as  in  10,  add  HKC204  —  pres- 
ence or  absence  of  Ca,  Sr,  Ba,  (Mg),  Zn,  Cd,  Sb,  Sn,  Pb,  U,  Mn, 
Fe,Ni,  Co,Cu,Ag,  (Hg),  (Cr). 

12.  Test  for  the  acids. 

13.  From  the  solubility  tests,  optical  behavior  and  above 
reactions  make  a  list  of  the  elements  which  are  probably  absent 
and  proceed  to  make  identity  tests  for  those  which  can  be 
present.  In  all  tests  applied  observe  carefully  the  precautions, 
notes  and  interferences  given  in  the  discussion  of  the  test. 

14.  Make  special  tests  for  silicates,  titanates,  etc. 

THE   COMMON   ACIDS. 

The  detection  of  the  acid  radicals  (anions)  by  microchemical 
reactions  is  much  more  difficult  than  the  identification  of  the 
bases  (cations).  This  is  largely  due  to  the  relatively  high 
concentrations  employed,  to  rapid  evaporation  taking  place 
during  manipulations  and  to  the  fact  that  through  the  addition 
of  reagents  many  complex  salts  are  formed  of  lower  solubilities 
than  those  originally  existing  in  either  unknown  or  reagent. 

In  the  elementary  course  whose  outline  is  covered  by  this 
textbook  the  identification  of  the  acid  radicals  in  simple  salts 
or  simple  mixtures  alone  is  undertaken.  With  materials  of  this 
nature  the  qualitative  analysis  is  comparatively  easy  and  no 
elaborate  directions  or  schemes  of  procedure  are  necessary. 
Most  of  the  tests  for  the  acids  have  already  been  studied  and  it 
is  merely  necessary  in  most  cases  to  reverse  the  test  for  the 
bases  to  enable  us  to  properly  identify  the  acids. 

The  behavior  of  the  crystals,  obtained  in  a  test,  toward  polar- 
ized light  will  be  found  to  be  of  great  value  in  identifying  the 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ACIDS      369 

salts  present  in  a  mixture.  The  student  should  have  acquired 
therefore,  early  in  the  course,  the  habit  of  examining  his  prep- 
arations between  crossed  nicols.  Proceeding  in  this  manner  in 
connection  with  the  qualitative  tests  we  can  usually  determine 
the  true  nature  of  the  salts  present. 

In  testing  for  the  acids  it  is  essential  that  the  student  shall 
always  examine  the  preparations  before  they  evaporate  to  dry- 
ness and  that  he  shall  carefully  observe  the  various  precautions 
which  have  been  given  in  the  discussion  of  the  various  tests  for 
the  bases. 

When  dealing  with  an  unknown  substance  first  spread  out 
a  little  of  the  dry  material  upon  a  slide  and  examine  it  with 
a  low  power.  If  the  material  is  not  homogeneous,  endeavor 
to  pick  out  particles  of  its  different  components,  using  a  plat- 
inum wire  or  glass  rod.  Then  work  upon  each  component 
separately. 

Try  the  solubility  in  water,  acids,  etc. 

Test  the  reaction  toward  litmus-silk  (Method  VIII,  page  260) 
or  other  indicator. 

If  the  material  is  crystallizable,  make  observations  as  to  its 
probable  crystal  system.  Test  the  crystals  between  crossed 
nicols. 

Finally  make  rough  estimations  of  the  refractive  indices  by 
the  immersion  method  or  make  melting-point  determinations,  or 
both,  if  possible. 

For  convenience  in  microchemically  testing  for  the  acids  we 
may  make  use  of  the  following  slight  modification  of  the  Bunsen- 
Treadwell  classification  of  the  acids,  based  upon  the  behavior 
of  their  salts  toward  silver  nitrate,  and  toward  barium  chloride, 
in  neutral  and  in  nitric  acid  solutions. 

In  case  a  free  acid  is  to  be  dealt  with  it  is  best  to  add  ammo- 
nium hydroxide  in  slight  excess  and  drive  off  the  excess,  after 
neutralization,  by  evaporation  to  dryness.  Then  proceed  as 
follows: 

I.  To  a  drop  of  the  moderately  concentrated  aqueous  solution  of  the  un- 
known apply  a  drop  of  concentrated  solution  of  silver  nitrate  by  Method  /, 
page  251. 


370  ELEMENTARY   CHEMICAL  MICROSCOPY 

A.  No  precipitate  is  produced  and  no  crystalline  deposit  is  ob- 
tained until  the  drop  concentrates  through  spontaneous  evapo- 
ration.    See  I.  A,  below. 

B.  A  colored  precipitate  is  produced.     See  I.  B,  below. 

C.  A  white  or  colorless  precipitate  is  produced.     See  page  371. 
After  a  few  seconds. apply  a  small  drop  of  nitric  acid  (1  :  3)  to 

the  zone  of  precipitate. 

1.  The  precipitate  dissolves  in  whole  or  in  part.  If  only  in 
part,  decant  the  solution  and  apply  a  fresh  drop  of  nitric  acid 
to  the  residue,  to  ascertain  if  the  unknown  consists  of  a  mixture 
of  both  soluble  and  insoluble  silver  salts. 

2.  The  precipitate  is  unajfected. 

II.  To  another  drop  of  the  dilute  aqueous  solution  add  a  drop  of  barium 
chloride  solution.     See  page  251. 

A.  No  precipitate  results.     See  page  372. 

B.  An  amorphous,  granular  or  crystalline  precipitate  is  pro- 
duced.    See  page  372. 

1 .  The  precipitate  is  soluble  in  whole  or  in  part  in  nitric  acid. 

2.  The  precipitate  is  insoluble  in  nitric  acid. 

III.  To  a  drop  of  the  dilute  aqueous  solution  of  the  unknown  material  add 
a  drop  of  nitric  acid.     A  granular  or  amorphous  precipitate  results.    See  page 

373- 


I.   A.   No  Precipitate  with  Silver  Nitrate. 

Chlorate. 

Fluoride;  silicofluoride.1 

Nitrate. 

Perchlorate.1 

Sulphate.1 

I.    B.    The  Precipitate  is  Colored  (by  Reflected  Light). 

Arsenate.  Red,  brown  or  thick  crystals  black. 

Arsenite.  Yellow. 

Chromate,  bichromate.  Red,  brown  or  black. 

1  Crystals  separate  slowly   from  moderately  concentrated  solutions  or  even 
from  dilute  solutions  on  long  standing. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ACIDS      371 


Ferricyanide. 

Iodide. 

Iodate. 

Manganate,  permanganate. 

Nitrite. 

Phosphate. 
Sulphide. 


Yellowish-red,  or  brownish-red. 

So  faintly  yellow  as  to  appear  white. 

So  faintly  yellow  as  to  appear  white. 

Violet. 

Colorless    unless   in    masses,    then 

greenish. 
Yellow. 
Black  or  brown. 


I.   C.i.    The  White  or  Colorless  Precipitate  Dissolves. 


Appearance  of  the  precipitate  before  the 
nitric  acid  is  applied. 

Crystalline;  prisms  and  plates. 

Granular. 

Amorphous  or  granular. 

Dense  amorphous. 

Granular  or  crystalline  in  tiny  stars  or  tine 
needles.     Difficultly  soluble  in  HN03. 

Long  slender  needles. 

Granular  or  crystalline;  short  stout  prisms, 
rhombs  or  hexagons. 

Prisms,  rhombs  and  crystallites. 

Granular  or  crystalline;  prisms. 

Amorphous  becoming  crystalline;  crystallites 
and  prisms. 

Dense  amorphous,  or  granular,  white  changing 
to  yellow,  red-brown  or  dark  brown  due  to 
formation  of  silver  sulphide.  When  much 
sulphur  separates  the  precipitate  may  ap- 
pear to  be  insoluble  in  HN03. 


I.   C.  2.    The  Colorless  Silver  Salt  is  Insoluble  in  Nitric  Acid. 


Salts. 

Acetates. 

Borates. 

Carbonates. 

Cyanates. 

Iodates. 

Nitrites. 
Oxalates. 

Sulphates. 

Sulphites. 

Tartrates. 

Thiosulphates. 


Chloride. 
Bromide. 
Iodide. 


Hypochlorite. 

Ferrocyanide.1 

Sulphocyanate. 


1  Turns  yellowish  red  or  brown  when  drop  of  nitric  acid  is  applied. 


372 


ELEMENTARY   CHEMICAL  MICROSCOPY 


II.    A.   No   Immediate 

Precipitate   is   Obtained   with  Barium 

Chloride. 

Acetate. 

Arsenate.1 

Ferrocyanide.1 

Borate.1 

Iodide. 

Bromide. 

Nitrate. 

Chlorate. 

Nitrite. 

Chloride. 

Oxalate.1 

Cyanide. 

Cyanate. 

Ferricyanide. 

II.   B.  i.   Barium  Chloride  gives  a  Precipitate  Soluble  in  Nitric 

Acid.2 

^altc 

Appearance  of  the  precipitate  before  the 

Odllb. 

nitric  acid  is  applied. 

Arsenites. 

Amorphous. 

Carbonates. 

Amorphous  or  granular;    becoming 

crystalline. 

Chromates,  bichromates. 

Yellow  granular,  or  crystalline,  only 

slowly  soluble  in  nitric  acid. 

Cyanates. 

From    concentrated    solutions,    in 

prisms. 

Fluorides. 

Granular. 

Iodates. 

Stars   and   dendrites.     Only   slowly 

soluble. 

Phosphates. 

Amorphous  or  granular. 

Sulphites. 

Granular  or  crystalline. 

Tartrates. 

Granular. 

II.   B.  2.    The  Precipitate  obtained  with  Barium  Chloride   is 
Insoluble  in  Nitric  Acid. 
Silicofluoride. 
Sulphate. 

Chromate,  bichromate  and  iodate  precipitates  are  only 
slowly  soluble  in  nitric  acid. 

1  With  concentrated  solutions  of  these  salts  barium  chloride  will  give  a  slowly 
formed  crystal  deposit. 

2  Concentrated  nitric  acid  precipitates  barium  nitrate  in  large  colorless,  iso- 
metric crystals. 


MICROCHEMICAL   REACTIONS   OF   THE   COMMON    ACIDS     373 

III.  Nitric  Acid  produces  an  Amorphous  or  Granular  Pre- 
cipitate. 

Molybdate. 

Silicate. 

Tungstate. 

Titanate. 

Zirconate. 

Note.  —  It  must  be  remembered  that  the  addition  of  strong 
nitric  acid  will  cause  a  crystalline  precipitate  in  the  case  of  many 
salts  of  low  solubility. 


A  somewhat  better  scheme  of  separation  of  the  acids  has  been 
proposed  by  C.  G.  Hinrichs1  based  upon  the  behavior  of  their 
salts  toward  acetic  and  sulphuric  acids  when  heated. 

Group  I.  —  Salts  which  when  heated  with  strong  acetic  acid 
are  decomposed  and  certain  components  are  volatilized. 

Carbonate  (C02). 
Cyanide  (HCN). 
Hypochlorite  (to  CI). 
Hyposulphite  (S02). 
Nitrite  (oxides  of  N). 
Sulphide  (H2S). 
Sulphite  (S02). 
Group  II.  —  Salts  which  when  heated  with  strong  sulphuric 
acid  are  decomposed  and  certain  components  are  volatilized. 

Acetate  (HC2H302). 

Borate  (B(OH)3). 

Bromide  (HBr). 

Chlorate  (HC103). 

Chloride  (HC1). 

Cyanate  (C02  and  NH3,  latter  forms  (NH^SC^). 

Ferrocyanide  (HCN). 

Ferricyanide  (HCN). 

Iodide  (HI). 

Nitrate  (HN03). 

1  Hinrichs,  Microchemical  Analysis,  p.  116,  St.  Louis,  1904. 


374  ELEMENTARY   CHEMICAL  MICROSCOPY 

Group  III.  —  Non-volatile  with  sulphuric  acid. 

Arsenate. 

Arsenite. 

Chromate,  bichromate. 

Manganate. 

Permanganate. 

Phosphate. 

Sulphate. 

The  separation  by  the  above  method  may  be  carried  out  as 
described  under  Distillation,  page  245. 

ACETATES. 

a.  With  Silver  Nitrate  in  concentrated,  approximately  neutral 
•solution,  pearly  scale-like  crystals  of  silver  acetate  are  obtained. 
Later  these  develop  into  long  thin  prisms  with  more  or  less 
irregular  sides  and  ends.  Those  in  which  six  edges  are  de- 
veloped give  terminal  angles  a  trifle  over  90  degrees,  and  extinc- 
tion almost  parallel  with  their  length  (extinction  angle  8  degrees). 
To  confirm  the  test  distil  a  portion  acidified  with  phosphoric 
acid.  In  the  absence  of  phosphoric  acid,  sulphuric  acid  may 
be  employed. 

b.  With  Mercurous  Nitrate  added  to  concentrated  solutions. 
Colorless  plates  and  prisms;  the  thin  six-sided  prisms  have  their 
terminal  angles  equal  to  100  degrees  and  exhibit  parallel  ex- 
tinction. 

c.  With  Sodium  Chloride  and  Uranyl  Nitrate  in  approximately 
neutral  solutions.  Sodium  uranyl  acetate  is  obtained.  See 
Sodium,  Method  A,  page  274.  Add  the  uranyl  nitrate  to  the 
drop  of  unknown,  and  draw  this  solution  across  the  dry  film  of 
sodium  chloride. 

ARSENATES. 

a.  With  Silver  Nitrate.    See  Silver,  page  337 ;  Arsenic,  page  351. 

b.  With  Zinc  Acetate  and  Ammonium  Chloride  in  Ammoniacal 
Solution.     See  Magnesium,  page  306. 

ARSENITES. 

a.  With  Silver  Nitrate.     See  Arsenic,  page  351. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ACIDS      375 

BORATES. 

a.  With  Ammonium  Fluoride  in  Dilute  Hydrochloric  Acid  Solu- 
tion. Add  to  the  drop  on  a  celluloid  slip  NaCl,  or  BaCl2,  then 
the  reagent,  then  a  trace  of  HC1.     See  Sodium,  page  279. 

Precautions.  —  Silicon,  titanium  and  zirconium  must  be  ab- 
sent.    The  test  drop  must  be  moderately  concentrated. 

b.  Test  with  a  Turmeric  Linen  Fiber.     See  page  261. 

BROMIDES. 

a.  Staining  Starch  Yellow. 

To  a  drop  of  the  solution  to  be  tested  add  a  trace  of  dilute 
sulphuric  acid,  warm  gently.  Cool.  Add  a  very  little  potato 
starch,  just  enough  to  give  a  few  granules  in  the  center  of  the 
drop.  Introduce  at  the  center  of  the  drop  a  small  crystal  of 
ammonium  persulphate.  Bromine  is  set  free  and  colors  the 
starch  granules  yellow.  If  iodides  are  present  the  starch  will 
be  colored  blue  or  violet. 

Too  long  and  too  high  heating  will  result  in  the  loss  of  hydro- 
bromic  acid. 

The  preparation  must  be  cool  when  the  starch  is  added, 
otherwise  the  granules  will  be  destroyed. 

The  preparation  must  be  examined  at  once,  otherwise  the 
yellow  color  will  have  disappeared. 

b.  Silver  bromide  (and  silver  chloride)  is  soluble  in  ammonium 
hydroxide;    silver  iodide  is  not. 

CARBONATES. 

a.  Characterized  by  Effervescence  with  hydrochloric  or  sulphuric 
acid.  Gas  bubbles  visible  in  gelatin.  See  page  263.  Cyanates 
give  a  similar  reaction,  carbon  dioxide  being  formed  by  the  reac- 
tion between  cyanate  and  acid. 

b.  In  Solutions  of  Carbonates,  Lead  Acetate  produces  character- 
istic crystals  of  lead  carbonate. 

c.  To  test  the  character  of  the  gas  given  off,  place  in  the  distilling 
apparatus,  Fig.  130,  page  245,  exposing  a  drop  of  lead  acetate 
to  the  vapors. 


376 


ELEMENTARY   CHEMICAL   MICROSCOPY 


CHLORIDES. 

a.  With  Silver  Nitrate. 

b.  With  Lead  Nitrate. 


See  Silver,  page  331. 
See  Lead,  page  325. 


CHLORATES. 

a.  Test  the  material  with  Rubidium  Chloride  and  a  little  Potas- 
sium Permanganate  to  be  sure  perchlorates  are  absent  (see  Ex- 
periment a,  Method  IX,  page  262).  TJien  Convert  into  Perchlo- 
rates as  follows: 

Dissolve  a  little  of  the  material  in  a  drop  of  water  at  the  corner 
of  an  object  slide,  evaporate  to  dryness.  Add  a  drop  of  sul- 
phuric acid,  evaporate  to  dryness  and  heat  until  white  fumes 
escape.  Add  a  second  drop  of  acid  and  heat  until  the  excess 
of  sulphuric  acid  has  been  driven  off.  Cool.  Add  a  tiny  drop 
of  potassium  permanganate  (just  sufficient  to  color  the  drop) 
and  a  crystal  of  rubidium  chloride.  Allow  to  stand  for  a  short 
time  and  examine.  Characteristic  crystals  of  rubidium  per- 
chlorate  will  separate,  colored  pink  or  violet  through  adsorption 
of  the  permanganate. 

The  chlorate  is  only  partially  converted  into  the  perchlorate, 
hence  this  test  is  not  always  successful,  and  is  of  little  value  in 
complex  mixtures. 

CHROMATES;   BICHROMATES. 

a.  Test  with  Silver  Nitrate  in  nitric  acid  solution.  See  Silver, 
page  334;   Chromium,  page  357. 

b.  Test  with  Strontium  Acetate.     See  page  301. 

c.  Bichromates  give  no  separation  of  crystals  with  Manganous 
Sulphate;  Chromates  do.     See  Manganese,  page  360. 

CYANIDES. 

a.  Place  the  material  in  the  glass  crucible  of  apparatus,  Fig. 
130,  page  245;  moisten  with  dilute  sulphuric  acid,  cover  with  a 
slide  bearing  a  drop  of  silver  nitrate.  If  no  tiny  prismatic 
crystals  are  obtained  and  no  clouding  of  the  silver  nitrate, 
cyanides  are  absent.  If  a  clouding  of  the  drop  results,  make  a 
fresh  test,  this  time  substituting  for  the  sulphuric  acid,  a  saturated 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ACIDS       377 

solution  of  primary  sodium  carbonate;    hydrocyanic  acid  will 
be  set  free  and  will  give  a  characteristic  silver  cyanide. 

b.  Set  free  the  vapors  of  the  acid  and  expose  to  them  a  drop 
of  sodium  picrate.     A  blood  red  solution  results. 

CYANATES. 

a.  To  a  drop  of  concentrated  solution  add  at  the  center,  a 
tiny  crystal  of  cobalt  acetate.  The  crystal  will  be  immediately 
surrounded  by  a  deep  blue  colored  zone  and  a  blue  amorphous 
precipitate.  The  blue  zone  increases  in  diameter  and  eventually 
may  reach  the  circumference  of  the  drop.  Upon  evaporation 
deep  blue  tetragonal  dendrites,  and  tabular  and  prismatic  crys- 
tals of  a  compound  corresponding  to  the  formula  K2Co(CNO)4 
will  appear.  Note  that  to  obtain  this  compound  the  cyanate 
must  be  in  excess.  With  sulphocyanates  tested  thus  a  deep 
blue  liquid  is  obtained  on  evaporation,  but  the  blue  dendrites 
which  may  separate  have  a  different  habit. 

Cyanides  yield  no  blue,  but  a  brown  color  instead.  Even  a 
small  amount  of  cyanide  will  prevent  the  blue  zone,  but  the 
crystal  will  be  blue  surrounded  by  a  yellow  or  brown  zone. 

b.  Treat  a  drop  with  dilute  sulphuric  acid  in  the  distilling 
apparatus,  Fig.  130,  page  245.  Evaporate  very  gently  almost 
to  dryness;  add  a  few  fibers  of  freshly  ignited  asbestos  and  pro- 
ceed to  test  for  ammonia.  See  Ammonium,  Method  A ,  page  286. 
With  sulphuric  acid  cyanates  yield  carbon  dioxide  and  ammo- 
nium sulphate. 

Precaution.  —  Always  make  a  blank  test  upon  the  reagents 
to  be  sure  of  their  freedom  from  ammonium  salts. 

FERRIC  YANIDES. 

a.  Give  of  Vapors  when  heated  with  sulphuric  acid  which 
produce  silver  cyanide.     See  Cyanides,  a,  page  377. 

b.  To  the  test  drop  add  sodium  acetate,  then  apply  a  solution 
of  Benzidine  Hydrochloride1  by  Method  /,  page  251.  Light  blue 
prisms  and  stars  will  soon  appear. 

Ferrocyanides  do  not  give  this  reaction. 

c.  Give  no  color  with  dilute  solutions  of  pure  Ferric  Salts. 

1  Behrens,  Z.  anal.  Chem.,  43,  432- 


378  ELEMENTARY   CHEMICAL  MICROSCOPY 

FERROCYANIDES. 

a.  Give  a  Blue  Precipitate  with  Salts  of  Iron  and  a  brown  one 
with  salts  of  copper  in  acetic  acid  solution. 

b.  With  Quinoline  Hydrochloride  yield  upon  warming  cubical 
crystals. 

IODIDES. 

a.  To  a  drop  of  solution  add  dilute  sulphuric  acid,  a  little 
potato  starch  and  a  tiny  fragment  of  ammonium  persulphate. 
The  starch  is  turned  blue  or  violet  in  the  cold.     See  Bromides, 

page  375- 

b.  The  silver  nitrate  precipitate  is  insoluble  in  ammonium 
hydroxide;   distinction  from  chloride  and  bromide. 

c.  Yield  characteristic  hexagonal  plates  with  lead  nitrate. 
See  Lead,  page  323. 

IODATES. 

a.  Dissolve  in  water,  add  a  very  tiny  drop  of  dilute  sulphuric 
acid,  a  little  potato  starch  and  finally  a  crystal  fragment  of 
morphine  sulphate.  Iodine  is  set  free  and  the  starch  granules 
turn  blue  or  violet. 

Iodides  do  not  give  this  reaction;  nor  will  iodates  give  reaction 
a  under  iodides. 


NITRATES. 

a.  With  Nitron  Sulphate  in  Acetic  Acid  Solution.  Apply  the 
reagent  by  Method  7,  page  251. 

There  is  immediately  formed  a  heavy  precipitate,  consisting  of 
masses  of  exceedingly  minute  needles.  In  a  few  seconds  sheaves 
of  acicular  prisms  appear  and  later  there  are  formed  long  thin 
prisms  with  square  ends,  giving  polarization  colors  and  parallel 
extinction.  Nitron  nitrate  has  a  very  low  solubility  even  in 
warm  water,  hence  the  reaction  is  a  delicate  one.  The  sheaves 
of  white  crystals,  appearing  brownish  by  reflected  light,  are 
characteristic. 

In  dilute  solutions  none  of  the  salts  of  the  common  acids  inter- 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ACIDS      379 

fere  save  iodides  and  bichromates.  With  these  salts  there  may 
be  obtained  crystals  which  closely  resemble  the  nitrate  but  these 
crystals  disappear  upon  even  gentle  warming;  nitron  nitrate  will 
not. 

From  concentrated  solutions  there  may  be  obtained  under 
favorable  conditions,  precipitates  with  chlorates,  perchlorates, 
phosphates,  chromates,  bichromates,  iodides,  ferro-  and  ferri- 
cyanides,  oxalates  and  tartrates,  but  in  no  case  in  dilute  solu- 
tions with  gentle  warming  should  there  be  any  difficulty  in 
differentiating  between  such  precipitates  and  the  crystals  ob- 
tained with  nitrates. 

NITRITES. 

a.  With  Silver  Nitrate  there  is  obtained  a  felted  mass  of  fine 
needles  with  long  acicular  prisms  at  the  outer  edge  of  the  mass, 
changing  into  short  stout  prisms  with  imperfectly  developed 
ends.  These  crystals  are  colorless  under  the  microscope  and  do 
not  show  their  greenish  tint  until  viewed  in  masses  by  reflected 
light. 

b.  With  Potassium  Iodide  and  Starch.  Add  to  the  drop  to  be 
tested  a  crystal  of  potassium  iodide,  then  a  little  potato  starch 
and  finally  a  trace  of  dilute  sulphuric  acid.  The  hydroiodic 
acid  set  free  by  the  acid  is  oxidized  by  the  nitrous  acid;  iodine 
is  liberated  and  stains  the  starch  blue  or  violet  or  black. 

Always  test  the  potassium  iodide,  with  starch  and  dilute  sul- 
phuric acid,  to  ascertain  its  purity  and  to  be  certain  that  no 
appreciable  blueing  of  the  starch  takes  place  with  the  reagents 
alone. 

Only  traces  of  .iodine  are  liberated  from  iodide  when  treated 
with  a  crystal  of  morphine  sulphate  as  described  under  iodates, 
page  378. 

OXALATES. 

a.  With  Strontium  Acetate.  See  Calcium,  page  291;  Stron- 
tium, page  295. 

b.  With  Silver  Nitrate  or  Lead  Nitrate.    See  Calcium,  page  291. 


38o 


ELEMENTARY   CHEMICAL   MICROSCOPY 


PHOSPHATES. 

a.  To  the  drop  to  be  tested,  add  a  drop  of  Nitric  Acid.  Then 
apply  a  drop  of  Ammonium  Molybdate  by  Method  /,  page  251. 
Warm  gently.  Phosphates  yield  a  yellow  precipitate  appearing 
amorphous  under  the  microscope  unless  a  magnification  of  over 
200  is  employed.  A  similar  reaction  will  be  obtained  if  silico- 
molybdates  or  arseno-molybdates  are  formed. 

This  reaction  is  of  value  if  arsenic  and  soluble  silicates  are 
absent  and  as  indicating  whether  much  or  little  phosphate  is 
present.     If  a  heavy  precipitate  is  obtained,  apply  test  b. 

b.  To  the  Ammoniacal  Solution  add  Ammonium  Chloride  and 
Magnesium  Acetate,  proceeding  as  described  under  Magnesium, 
page  305.     Arsenates  must  be  absent. 

Note.  —  Phosphates  frequently  interfere  with  the  detection 
of  certain  bases  and  must  be  removed  before  reliable  reactions 
can  be  obtained;  their  removal  may  be  accomplished  by  means 
of  tin  in  acid  solution.  Acidify  with  nitric  acid,  add  a  few  tiny 
bits  of  pure  tin-foil  and  as  soon  as  the  reaction  has  ceased,  heat 
to  boiling.  Cool  and  extract  the  material  with  dilute  nitric 
acid. 


SILICATES. 

a.  Treat  the  material  upon  a  celluloid  object  slide  with  am- 
monium fluoride,  sodium  chloride  and  sulphuric  acid.  Sodium 
silicofluoride  is  formed.  See  Sodium,  page  278.  Boron,  zirco- 
nium and  titanium  must  be  absent. 


SULPHATES. 

a.  To  the  drop  add  a  trace  of  Nitric  Acid,  then  a  drop  of  Cal- 
cium Acetate  by  Method  /,  page  251.  Characteristic  needles  or 
prisms  of  calcium  sulphate  result.     See  Calcium,  page  288. 

b.  To  the  drop  add  a  trace  of  Potassium  Chromate,  a  trace  of 
Nitric  Acid  and  a  drop  of  Silver  Nitrate.  Characteristic  crystals 
of  silver  sulphate  will  be  obtained,  stained  yellow  through  solid 
solution  of  the  silver  chromate.     See  Silver,  page  335. 


MICROCHEMICAL  REACTIONS  OF  THE  COMMON  ACIDS      381 

SULPHITES,   THIOSULPHATES. 

a.  To  a  drop  of  a  solution  of  potassium  iodate  add  a  little 
potato  starch  and  a  small  drop  of  dilute  sulphuric  acid.  Ex- 
amine to  see  that  no  iodine  has  been  set  free.  Add  a  fragment 
of  the  unknown.     The  starch  is  colored  blue. 

b.  To  a  moderately  concentrated  drop  of  copper  sulphate 
apply  a  drop  of  a  solution  of  the  unknown  by  Method  7/7  A, 
page  254.  Warm  gently  —  sulphites,  if  pure  and  undecom- 
posed,  yield  at  the  most  only  a  faint  cloudiness  -  -  thiosul- 
phates  give  a  brown  precipitate  of  copper  sulphide  and  around 
the  circumference  of  the  drop  lemon-yellow  crystals  of  copper 
thiosulphate. 

SULPHIDES. 

a.  The  Silver  Nitrate  Precipitate  was  Black. 

b.  Place  a  drop  of  solution  or  fragment  of  solid  in  the  distilling 
apparatus,  cover  with  a  slide  holding  a  tiny  drop  of  silver  nitrate 
and  one  of  lead  acetate  side  by  side.  Raise  the  cover  and  care- 
fully run  in  a  drop  or  two  of  dilute  hydrochloric  acid.  Cover 
quickly  and  allow  to  stand.     Both  drops  turn  black. 

c.  Proceed  exactly  as  in  b  but  invert  over  the  crucible  a  slide 
carrying  a  drop  of  sodium  nitroprusside.  A  beautiful  purple 
color  results. 

SULPHOCYANATES. 

a.  Give  a  Blood-red  Color  with  dilute  Ferric  Chloride. 

b.  Add  Mercuric  Chloride  and  Zinc  Sulphate.  There  will  be 
obtained  the  double  sulphocyanate  of  mercury  and  zinc.  See 
Zinc,  page  308;  Copper,  page  340.  Add  a  trace  of  copper  and 
increase  the  delicacy  of  the  reaction. 

TARTRATES. 

Note.  —  Before  testing  for  tartrates  always  neutralize  any  free 
mineral  acid  present. 

a.  By  means  of  Calcium  Acetate. 

The  solution  may  be  neutral  or  acidified  with  acetic  acid. 


382 


ELEMENTARY   CHEMICAL  MICROSCOPY 


Large,  colorless,  well-formed,  highly  refractive  crystals  are 
obtained. 

The  solution  to  be  tested  must  be  concentrated,  otherwise  the 
calcium  tartrate  will  not  separate  save  on  long  standing.  Ex- 
posure to  alcohol  vapors  (Method  VI,  page  257)  will  hasten  the 
formation  of  a  crystal  deposit. 

Magnesium  salts  greatly  retard  the  separation  of  crystals  of 
calcium  tartrate. 

b.  With  Potassium  Salts,  tartrates  yield  characteristic  color- 
less, highly  refractive,  orthorhombic,  short,  stout  prisms  of  the 
primary  salt  KHC4H4O6. 

c.  With  Silver  Nitrate. 

A  granular  precipitate  only  is  obtained  unless  in  very  dilute 
solution,  then  there  will  be  obtained  tiny  squares  and  rectangles 
and  short,  stout  prisms  giving  a  six-sided  outline. 

Most  other  acids  interfere  with  the  detection  of  tartrates  by 
means  of  the  silver  salt. 


CHAPTER  XV. 

PREPARING  OPAQUE  OBJECTS  FOR  THE  MICROSCOPIC 
STUDY  OF  INTERNAL   STRUCTURE. 

In  order  that  alloys  and  many  other  similarly  constituted 
materials  may  be  properly  studied  and  their  internal  struc- 
tures ascertained  it  is  usually  essential  that  large  or  small  pieces 
be  ground  down  to  a  plane  surface  which  may  be  so  placed 
under  the  microscope  as  to  lie  at  right  angles  to  the  optic  axis 
of  the  instrument.  It  is  further  necessary  that  this  plane  sur- 
face shall  be  so  smooth  as  to  show  no  striations  due  to  grinding, 
otherwise  these  parallel  or  irregular  streaks  will  confuse  the 
observer.  Removal  of  the  streaks  is  accomplished  by  polishing 
or,  in  other  words,  grinding  with  an  abrasive  so  fine  that  the 
scratches  made  are  so  close  together  and  so  shallow  that  they 
will  not  be  resolved  by  the  objectives  used  in  the  microscopic 
examination.  If  these  polished  specimens  are  subjected  to  the 
action  of  various  solvents,  it  will  be  found  that  in  non-homo- 
geneous materials,  certain  components  are  easily  dissolved  and 
certain  others  are  resistant.  The  specimen  thus  treated,  is  said 
to  have  been  etched,  and  when  the  etched  surface  is  examined  a 
more  or  less  marked  crystalline  structure  is  visible.  Through 
the  judicious  selection  of  the  proper  etching  liquids  we  are  able 
to  bring  into  view  different  components  or  phases  and  thus  trace 
the  changes  in  structure  through  changes  in  percentage  compo- 
sition, or  through  changes  in  the  temperatures  to  which  the 
specimens  have  been  submitted. 

Or  instead  of  submitting  the  polished  surface  to  the  action  of 
a  corrosive  liquid,  we  can  rub  it  upon  a  thick,  soft  cloth  charged 
with  a  fine  abrasive  powder.  The  softer  components  will  thus 
be  more  rapidly  worn  away  than  the  harder;  again  we  obtain 
evidence  of  a  more  or  less  marked  crystalline  structure.  The 
specimen  is  no  longer  spoken  of  as  having  been  etched,  but 

383 


384  ELEMENTARY   CHEMICAL   MICROSCOPY 

is  said  to  have  been  polished  in  relief.  Since  in  almost  all  the 
materials  commonly  studied  we  deal  with  components  differing 
in  hardness,  it  is  exceedingly  difficult  to  obtain  polished  speci- 
mens which  do  not  exhibit  some  relief  polishing.  Practice  and 
a  light  touch  are  the  only  effective  preventives. 

The  wearing  or  cutting  off  of  irregularities  so  as  to  obtain  a 
flat  surface  is  termed  roughing.  Roughing  is  most  easily  accom- 
plished by  holding  the  specimens  against  rapidly  revolving  ab- 
rasive wheels. 

The  most  useful  American  abrasive  wheels  are  emery,  co- 
rundum, alundum,  crystalon  and  carborundum.  Emery  and 
corundum  are  natural  products,  while  alundum,  crystalon  and 
carborundum  are  products  of  the  electric  furnace;  the  first  three 
mentioned  consist  of  crystallized  alumina,  the  last  two  consist 
of  crystalline  carbide  of  silicon.  Of  these,  emery  cuts  or  wears 
away  specimens  the  least  rapidly,  crystalon  and  carborundum 
the  most  rapidly. 

All  three  steps,  grinding,  polishing  and  etching,  require 
patience,  practice  and  a  certain  inherent  technical  skill.  Prac- 
tice, and  practice  alone,  will  enable  the  student  to  properly 
prepare  specimens.  The  selection  of  the  proper  sequence  of 
abrasives,  the  right  pressure  of  the  specimen'  against  the  grind- 
ing material,  the  rate  of  speed  or  motion  in  grinding  and  polish- 
ing all  enter  into  the  preparation  of  the  specimen.  No  specific 
directions  can,  therefore,  be  given,  but  merely  a  general  outline  of 
the  steps  to  be  taken  and  the  special  precautions  to  be  observed. 
So,  too,  in  the  etching  much  depends  upon  the  individual.  The 
proper  concentration  of  reagent  (which  differs  for  different  alloys 
of  the  same  type),  the  way  in  which  the  specimen  is  immersed  or 
submitted  to  the  action  of  the  reagent,  the  time  of  exposure, 
temperature  of  the  room  and  reagent,  thoroughness  of  removal 
of  the  etching  liquid  by  washing,  etc.,  each  enters  largely  into 
the  preparation  of  really  satisfactory  specimens  and  all  con- 
tribute to  the  elucidation  of  the  problem  or  to  the  confusion 
of  the  investigator. 

Grinding  wheels  are  made  from  powdered  abrasive  mixed 
with  a  suitable  binder,  pressed  into  moulds  and  fired  in  an  oven. 


PREPARING  OPAQUE  OBJECTS  385 

The  character  of  the  binder  and  the  degree  of  incipient  fusion 
characterizes  a  wheel  as  hard  or  soft.  The  degree  of  hardness 
or  softness  is  technically  spoken  of  as  the  grade  or  hardness  of 
the  wheel.  American  manufacturers  usually  indicate  the  grades 
of  their  wheels  by  letters  of  the  alphabet,  but  the  scale  of  hardness 
as  indicated  by  the  letters  is  by  no  means  uniform  with  different 
manufacturers.  Consequently,  a  letter  indicating  a  grade  can- 
not be  interpreted  without  reference  to  the  scale  of  hardness  of 
the  particular  firm  from  whom  the  wheel  was  obtained.  For 
example,  we  find  that  a  wheel  marked  U  may  be  "hard"  as 
supplied  by  one  firm,  but  if  we  purchase  a  U  grade  from  another 
firm  we  will  obtain  a  "very  soft"  wheel.  In  selecting  wheels 
for  grinding  specimens,  it  is  safe  to  be  guided  by  the  general 
rule  that  a  soft  wheel  will  cut  more  rapidly  and  deeper  than 
a  hard  one,  will  clear  itself  more  readily,  but  is  more  easily 
worn  away,  and  therefore  more  liable  to  be  spoiled.  The  soft 
wheels  as  a  rule  must  be  run  at  higher  speeds.  Hard  wheels 
on  the  other  hand  tend  to  glaze  over,  cause  more  heating  of  the 
specimen  and  often  yield  aggravated  cases  of  surface  films  or 
surface  flow  of  soft  components,  but  they  cut  slower,  hence  do 
not  so  deeply  score  or  furrow  the  specimen  through  injudicious 
pressure  and  may  be  employed  to  better  advantage  when  only 
low  speeds  are  available. 

Besides  the  grade  or  hardness  of  grinding  wheels  as  influencing 
their  suitability  for  certain  work,  the  diameter  and  the  uniform- 
ity of  the  individual  particles  employed  in  building  up  a  wheel 
must  be  taken  into  account.  The  size  of  the  component  par- 
ticles is  called  the  grain  or  grit.  Grain  is  obtained  in  manu- 
facturing by  screening  the  abrasive  powder.  The  number  of 
linear  meshes  to  the  inch  through  which  the  powder  will  pass 
is  the  grain  number  of  the  wheel.  For  example,  in  a  wheel 
marked  50,  the  component  particles  will  pass  through  a  sieve 
having  fifty  meshes  to  the  inch. 

The  grain  numbers  employed  by  different  manufacturers  are 
not  comparable  because  the  size  of  wire  employed  in  the  sieves 
used  for  the  grading  is  not  always  the  same.  Since  it  is  the 
number  of  linear  meshes  to  the  inch  and  not  the  diameter  of 


386 


ELEMENTARY   CHEMICAL   MICROSCOPY 


the  opening  that  is  recorded,  the  size  of  the  wire  greatly  influ- 
ences the  screened  product. 

Although  for  industrial  purposes  abrasive  wheels  may  be 
said  to  conform  closely  to  the  grade  and  grain  indicated  by  the 
manufacturer,  it  will  be  found  that  in  preparing  specimens  for 
microscopic  study,  wheels  are  not  easily  duplicated  and  if  we 
purchase  a  wheel  to  replace  one  accidentally  ruined  we  are  apt 
to  find  that  it  will  not  do  just  the  work  of  the  one  lost. 

Wheels  of  softer  grade  and  coarser  grain  (at  high  speeds)  can 
be  used  for  roughing  chilled  iron  and  steels,  —  hard  and  of  high 
tensile  strength,  —  than  for  material  like  brass  —  soft  and  of 
low  tensile  strength. 

No  single  type  of  wheel  as  to  grade  and  grain  will  answer  for 
all  purposes.  A  laboratory  in  which  a  great  variety  of  work  is 
to  be  done  will  therefore  require  a  series  of  wheels. 

Table  VI. 
CHARACTER  OF  ABRASIVE   WHEEL   REQUIRED. 


Alloy,  aluminum  type 

Alloy,  brass  type 

Alloy,  bronze  type 

Alloy,  nickel  type 

Iron,  cast 

Iron,  chilled 

Steel,  soft 

Steel,  hard 

Soft  porous  material 

Soft  friable  material 

Moderately  hard  compact  material . 
Very  hard  brittle  material 


Grain  or  grit. 


20  to  36 
20  to  46 
20  to  36 
20  to  36 
30  to  54 
20  to  46 
30  to  54 
60  to  100+ 

14  to  20 

46  to  80 

30  to  54 

100  to  180 


Grade  or  Hardness. 


Hard 
Hard 
Hard 
Medium-hard 
Medium-hard 
Medium-hard 
Medium 
Medium  to  medium- 
hard 

Hard 

Medium 

Medium 

Hard 


A  fairly  satisfactory  system  of  study  with  reference  to  the 
selection  of  wheels  for  different  materials  and  the  proper  speeds 
for  grinding  consists  in  examining  with  the  microscope  the 
roughed  surface  of  the  specimen  as  ground  under  different  con- 
ditions and  also  the  dust  or  particles  falling  from  the  wheel. 
These  particles  consist  of  material  torn  off  the  specimen  and 
particles  of  abrasive  and  binder.  The  character  of  the  dust  and 
the  furrows  upon  the  specimen  will,  with  a  little  experience, 


PREPARING  OPAQUE  OBJECTS  387 

indicate  at  once,  to  the  worker,  whether  he  is  employing  the 
proper  grade,  grain  and  speed.  It  is  strongly  urged  upon  the 
beginner  to  carry  out  experiments  in  this  manner  and  spend 
considerable  time,  if  possible,  in  ascertaining  just  what  different 
wheels  will  do  under  like  speeds. 

Table  VI  may  serve  as  a  rough  guide  to  the  selection  of  the 
wheel  which  will  prove  satisfactory  with  the  materials  indicated. 

If  the  grinding  room  equipment  is  limited  to  two  or  three 
wheels  it  is  evident  that  the  widest  range  of  applicability  will  be 
found  in  the  following  selection:  30  hard,  40  medium-hard,  and 
60  or  80  medium,  providing  a  sufficiently  high  speed  is  available. 

The  operating  speed  of  a  grinding  wheel  is  usually  expressed 
as  "surface  velocity"  in  feet  per  minute  in  order  that  wheels  of 
different  diameters  may  properly  be  compared. 

Surface  velocity  =  Diameter  wheel  fn  feet  X  3.1416  X  R.P.M.  of  arbor. 

Most  small  wheels  used  for  grinding  are  designed  to  run  with 
a  surface  velocity  of  from  2000  to  4000  feet  per  minute.  This 
requires  that  the  grinding  head  shall  rotate  at  the  rate  of  approx- 
imately 1800  to  3000  revolutions  per  minute  for  a  five  or  six  inch 
wheel,  if  the  data  given  in  Table  VI  are  followed.  For  slower 
speeds  it  will  be  necessary  to  select  finer  grains  and  harder 
grades.  In  order  to  permit  some  latitude  in  the  selection,  it  is 
best  to  have  the  grinding  head  and  driving  motor  provided  with 
cone  pulleys  or,  better  yet,  to  employ  a  shunt-wound  electric 
motor  and  rheostat  and  thus  obtain  a  variation  in  speeds. 

One  of  the  greatest  troubles  we  encounter  when  dealing  with 
abrasive  wheels  or  papers  or  powders  is  the  non-uniformity  of 
grain  size.  A  few  large  grains  present,  often  a  single  one  in  a 
small  area  of  the  grinding  surface,  will  so  deeply  scratch  the 
specimen  as  to  render  its  proper  preparation  almost  impossible. 
If  a  wheel  is  found  upon  trial  to  have  any  such  projecting  par- 
ticle the  wheel  should  be  abandoned  at  once,  and  never  be  em- 
ployed save  for  the  crudest  sort  of  grinding.  It  is  this  difficulty 
which  leads  many  workers  to  discard  abrasive  wheels  for  all  save 
the  roughest  dressing  of  a  specimen  and  use  only  laps  fed  with 
very  carefully  ground,  sifted  and  floated  abrasive  powders. 


388 


ELEMENTARY   CHEMICAL  MICROSCOPY 


Laps  may  be  either  horizontally  or  vertically  driven.  The 
beginner  will  find  that  satisfactory  surfaces  are  obtained  easier 
upon  the  horizontal  lap  but  it  is  open  to  the  objection  that  it 
does  not  readily  clear  itself  and  any  dust  or  dirt  falling  upon 
it  or  any  large  particle  of  abrasive  will  be  apt  to  deeply  groove 
the  specimen.  The  vertical  lap  on  the  other  hand  is  difficult  to 
keep  charged  with  pasty  abrasive  or  thin  suspensions  of  abrasive 
and  polishing  powders. 

In  the  case  of  soft  alloys,  facing  to  a  smooth  surface  is  most 
easily  accomplished  by  means  of  files,  rough  dressing  with  a 
10  or  1 2-inch  bastard  cut  file  and  passing  to  an  8  or  io-inch 
single  cut.  With  moderately  soft  materials  such  as  brass, 
laying  a  single  cut  mill  file  fiat  upon  the  work  bench  and  pushing 
the  specimen  down  the  file  against  the  cutting  edges  will  be 
found  to  yield  good  smooth  surfaces  with  less  practice  and  skill 
than  by  holding  the  specimen  in  a  vise  and  pushing  the  file. 
The  specimen  should  be  pushed  lengthwise  of  the  file  with  gentle 
pressure  until  it  reaches  the  tang  end,  then  lifted  off;  the  file 
turned  edgewise  and  struck  a  sharp  blow  upon  the  bench  to 
remove  filings,  again  laid  flat  and  the  specimen  again  laid  upon 
the  file  and  gently  pushed  toward  the  tang  end,  and  the  process 
repeated  until  a  small  plane  surface  is  obtained.  Specimens 
should  never  be  rubbed  back  and  forth  upon  an  abrasive  surface, 
for  it  is  then  almost  impossible  to  keep  the  striations  parallel, 
a  matter  of  not  a  little  importance. 

In  order  to  facilitate  smoothing  and  polishing,  the  edges  of  a 
specimen  should  always  be  slightly  beveled  or  rounded  during 
the  roughing.  Unless  this  precaution  is  taken  the  beginner  will 
find  it  difficult  to  avoid  cutting,  tearing  or  destroying  the  fabric 
carrying  the  polishing  powder. 

After  surfacing  with  wheel  or  file  the  specimens  are  smoothed 
upon  laps  fed  with  very  fine  abrasive  powder  or  upon  laps  or 
blocks  upon  which  abrasive  paper  has  been  smoothly  glued. 
Whenever  such  papers  are  employed  it  is  best  to  go  over  their 
surfaces  with  a  low-power  magnifier  and  reject  any  sheets  which 
show  isolated  large  particles  of  the  abrasive  covering.  Of  the 
fine-grained  abrasive  papers  tried  by  the  author,  the  French 


PREPARING   OPAQUE   OBJECTS  389 

"Hubert"  1  papers  are  the  best  and  most  uniform  of  grain.  The 
most  useful  are  numbers  0000,  000,  00,  o  arid  1,  the  last  named 
being  the  coarsest. 

General  Methods  for  Preparing  Hard  Specimens.  -  Grind  to  a 
plane  surface  upon  the  proper  wheel,  using  a  high  speed  and 
holding  the  specimen  so  that  it  just  barely  touches  the  rotating 
surface.  If  pressed  too  hard  against  the  wheel  there  will  be 
deep  scoring  and  too  much  heating.  Observe  great  care  to 
prevent  the  specimen  from  turning  in  the  fingers.  A  properly 
rough-ground  specimen  should  show  all  the  striations  parallel  and 
of  approximately  the  same  depth.  Next  bevel  or  round  the 
edges  of  the  specimen  around  the  ground  surface,  then  apply 
the  specimen  to  a  finer-grained  wheel  or  to  a  lap  fed  with 
finer-grained  powder,  grinding  so  that  the  striations  are  at  right 
angles  to  the  first.  Continue  grinding  until  when  examined  with 
a  low  magnification  no  vestiges  of  the  first  striations  remain. 
If  now  the  striations  are  very  shallow,  polishing  may  be  begun; 
if  not  shallow,  grind  with  a  third  finer  abrasive;  again  grinding 
at  right  angles  to  the  direction  last  taken  and  continuing  until 
all  trace  of  the  preceding  grinding  has  disappeared.  Polishing 
is  carried  out  in  like  manner,  using  finer  and  finer  powders 
moistened  to  a  pasty  consistency  with  water  or  oil  or  other 
suitable  vehicle.  When  oil,  vaseline  or  a  similar  vehicle  has  been 
employed  in  the  grinding,  especially  when  dealing  with  materials 
which  have  a  tendency  to  adsorb  the  grease,  as  for  example 
certain  rocks,  earthenwares,  terra-cottas,  porcelains,  cements 
and  concretes,  etc.,  it  will  be  found  that  polishing  proceeds  with 
far  greater  speeds  and  with  much  better  surfaces  when  the  pol- 
ishing powders  are  suspended  in  a  solvent  for  greases  and  oils, 
than  when  water  is  employed.  The  best  of  these  are  alcohols, 
ethers  and  light  petroleum  products  or  mixtures  of  them. 

With  each  change  in  fineness,  polish  at  right  angles  to  the 
former  motion.  Complete  the  polishing  with  the  finest  washed 
and  floated  rouge  or  floated  alumina  kept  well  moistened  upon 
soft  and  very  close-textured  broadcloth  stretched  upon  a  wooden 

1  These  imported  papers  can  be  obtained  from  Montgomery  and  Co.,  105 
Fulton  Street,  New  York  City. 


390 


ELEMENTARY   CHEMICAL  MICROSCOPY 


lap.  A  beautiful  mirror  surface  should  have  been  obtained  with 
no  signs  of  striations  when  examined  with  a  microscope  of  the 
same  power  as  will  be  employed  after  etching.  Wash  the  speci- 
men carefully,  and  dry  by  gently  pressing  with  lens  paper. 
Never  rub  when  drying  and  always  avoid  touching  the  polished 
surface  with  the  unprotected  ringers. 

If  oil  has  been  used  as  the  vehicle,  wash  first  with  gasoline  or 
benzene,  and  follow  with  alcohol  and  ether. 

General  Methods  for  Preparing  Soft  Specimens.  —  The  beginner 
should  never  attempt  to  grind  and  polish  soft  specimens  upon  a 
rotating  wheel  or  lap.  Even  the  roughing  is  best  done  with  a 
file  or  by  rubbing  upon  abrasive  paper  or  cloth  glued  upon 
blocks  of  wood.  Great  care  must  be  observed  in  rubbing  the 
specimen  so  that  it  shall  never  turn.  The  lines  of  abrasion  must 
be  kept  parallel.  Every  few  minutes  the  block  should  be  turned 
on  edge  and  struck  upon  the  bench  with  a  sharp  blow  in  order  to 
clear  it  from  loose  particles  and  dust;  if  this  is  not  done  deep 
scoring  of  the  surface  is  sure  to  follow.  When  passing  from  one 
abrasive  to  a  finer  one,  turn  the  specimen  to  a  position  at  right 
angles  to  the  other  and  rub  very  gently  until  every  trace  of  the 
former  scratches  has  disappeared.  The  polishing  is  carried  out 
in  the  same  manner  upon  close-textured  soft  cloth  stretched 
upon  blocks  and  covered  with  a  thin  paste  of  rouge  or  alumina, 
ending  up  with  the  finest  possible  floated  rouge.  It  will  be  found 
convenient  to  pass  from  a  grain  of  220  to  F,  to  FF,  to  FFF,  then 
to  ordinary  rouge  or  "rotten  stone"  and  finally  end  up  with  the 
finest  particles  obtained  by  washing  and  decanting  the  finest 
commercial  rouge.  Wash  and  dry  the  specimen  with  lens  paper. 
But  even  lens  paper  will  scratch  the  surface  of  soft  alloys  or 
other  soft  material. 

When  dealing  with  very  soft  materials,  after  washing  with 
water,  shake  off  the  last  drops  and  pour  absolute  alcohol  over  the 
polished  surface,  shake,  repeat  the  operation  and  then  remove 
the  last  traces  of  alcohol  with  a  few  drops  of  ether. 

Grinding  Hard  Friable  Material  like  Glass  or  Porcelain.  — 
Employ  lap  heads  of  block  tin  fed  with  emery  powder  and  water 
•or  turpentine.     Emery  does  not  cut  as  fast  as  carborundum, 


PREPARING   OPAQUE  OBJECTS  39 1 

crystalon  or  similar  abrasives,  but  also  does  not  so  deeply  score 
the  specimen  and  therefore  the  time  lost  in  grinding  is  usually 
gained  in  polishing. 

For  grinding,  the  lap  head  should  rotate  quite  slowly,  one  or 
two  hundred  revolutions  per  minute  being  the  maximum  for 
ordinary  work.  In  polishing  a  somewhat  higher  speed  may  be 
employed  with  advantage.  Polish  with  fine  rouge  and  complete 
the  finish  with  "putty  powder." 

Etching.  —  This  step  has  for  its  object  the  development  of 
the  crystalline  structure  of  the  specimen.  It  is  based  upon  the 
principle  of  submitting  the  polished  specimen  to  the  action  of  a 
corrosive  liquid  of  such  a  nature  as  to  dissolve  some  components 
more  rapidly  than  others. 

The  surface  to  be  treated  being  a  mirror  surface,  free  from  all 
striations,  it  follows  that  the  slightest  attack  by  an  etching 
liquid  will  be  easily  seen  by  means  of  the  microscope. 

Suppose,  for  example,  we  have  an  alloy  consisting  of  a  single 
crystalline  phase  and  an  eutectic.  Two  systems  of  attack 
would  reveal  the  nature  of  its  structure;  a  reagent  could  be 
employed  which  would  dissolve  the  eutectic  leaving  the  crys- 
talline phase  unattacked,  or  another  reagent  could  be  selected 
which  would  first  dissolve  the  crystals  leaving  the  eutectic. 
Whenever  it  is  possible,  specimens  should  be  etched  by  both 
systems,  for  then  the  probability  of  misinterpretation  of  appear- 
ances is  much  reduced.  The  development  of  the  structure  of  a 
specimen  so  as  to  render  its  microscopic  study  successful  requires 
considerable  practice. 

Small  specimens  are  grasped  in  rubber-tipped  or  cloth-covered 
(binding  tape)  forceps  and  dipped,  polished  surface  down,  or 
polished  surface  sidewise,  into  the  etching  liquid;  immediately 
removed,  washed  in  running  water,  dried  with  lens  paper  and 
examined.  If  the  structure  has  not  been  sufficiently  developed, 
it  is  again  dipped  and  again  washed  and  examined.  This  process 
is  repeated  until  the  etching  is  sufficiently  deep  to  make  the 
crystal  phase  or  phases  interpretable.  Too  long  immersion 
leads  to  uneven  etching,  to  crystal  sections  with  badly  eroded 
edges  and  often  to  serious  pitting.     With  many  of  our  etching 


392  ELEMENTARY    CHEMICAL   MICROSCOPY 

liquids  gases  are  formed;  the  tiny  gas  bubbles  clinging  to  the 
surface,  if  not  at  once  dislodged,  prevent  a  uniform  attack  and 
a  specimen  is  obtained  of  no  value  for  study.  The  only  course 
left  open  is  to  regrind  and  polish  anew. 

In  cases  where  much  gas  is  evolved  better  specimens  may 
often  be  obtained  by  dipping  a  small  wad  of  absorbent  cotton 
into  the  etching  liquid  and  gently  brushing  the  wet  cotton  upon 
the  surface,  washing  in  running  water  from  time  to  time.  In 
other  cases  stretching  a  piece  of  soft  clean  chamois  leather  upon 
a  board,  moistening  with  the  reagent  and  rubbing  the  specimens 
lightly  upon  this  surface  will  give  good  results. 

With  most  alloys  there  is  obtained  upon  the  completion  of  the 
polishing  a  thin  film  of  the  softer  components  more  or  less  com- 
pletely covering  the  surface,  due  to  surface  flow  during  the 
mechanical  treatment.  Not  infrequently  this  surface  film  is 
of  such  a  character  that  after  etching  the  appearance  of  the 
etched  surface  is  such  as  to  entirely  mislead  the  investigator. 
With  some  alloys  dipping  for  a  few  seconds  in  exceedingly  dilute 
acid  (sulphuric  is  best)  will  remove  the  film,  yet  not  appreciably 
etch  the  preparation.  This  often  essential  step  requires  con- 
siderable practice  in  order  to  duly  appraise  the  time  of  exposure 
to  the  acid  to  just  dissolve  the  surface  film  and  yet  not  attack 
the  polished  surface. 

The  following  are  a  few  of  the  most  generally  useful  of  etching 
reagents.  For  the  development  of  certain  specific  structures 
the  student  must  consult  the  literature  dealing  with  these  prob- 
lems. 

Ammonium  Hydroxide  +  Hydrogen  Peroxide.1  —  Immerse  the 
alloy  in  ammonium  hydroxide  diluted  to  such  a  strength  (i  .-4) 
that  the  alloy  is  not  rapidly  etched.  Add  hydrogen  peroxide 
from  a  pipette  dropwise.  This  method  gives  better  results 
than  mixing  the  reagents  before  the  specimen  is  immersed. 
Great  care  must  be  observed  to  avoid  too  rapid  an  attack  and 
too  deep  etching.     Excellent  for  alloys  high  in  copper. 

Ferric  Chloride.  -  -  Prepare  a  hot,  almost  saturated  solution 
of  ferric  chloride;  filter,  and  add  an  equal  volume  of  concentrated 
1  Ramsay,  Chem.  N.,  87  (1903),  291. 


PREPARING   OPAQUE   OBJECTS  393 

hydrochloric  acid.  For  use,  dilute  one  part  of  this  stock  solution 
with  twenty  parts  of  distilled  water.  If  upon  trial  the  etching 
is  too  energetic,  dilute  still  more;  if  not  energetic  enough,  add 
more  stock  solution. 

Useful  in  studying  bronzes  of  high  tin  content  and  copper 
alloys  in  general. 

Ferric  Chloride  +  Alcohol.  —  Robin1  prepares  this  reagent  as 
follows : 

Per  cent. 

Ferric  chloride 5 

Water 5 

Hydrochloric  acid 30 

Iso-amyl  alcohol 30 

Ethyl  alcohol 30 

The  etching  is  rapid  and  needs  careful  attention  to  prevent 
over  treatment,  one  to  three  minutes  being  the  average  exposure 
required. 

Valuable  in  studying  aluminum  bronzes  and  brasses. 

Hydrochloric  Acid  +  Absolute  Alcohol.  —  To  100  cubic  centi- 
meters of  absolute  alcohol  add  1  cubic  centimeter  of  concentrated 
hydrochloric  a'cid.  This  is  the  general  etching  reagent  of 
Martens  and  Heyn  for  all  iron-carbon  alloys.  Applicable  to  all 
specimens  but  must  be  used  with  care.  With  extra  hard  steels 
and  certain  alloy  steels  this  reagent  does  not  work  well.  In 
these  cases  Martens  suggests  the  nitric-alcohol  reagent.  Neither 
reagent  is  permanent,  but  must  be  freshly  prepared  for  use. 

Hydrochloric  -f-  Nitric  Acid.  —  Mix  three  parts  of  dilute  hy- 
drochloric acid  with  one  part  of  dilute  nitric  acid,  add  2  or  3 
drops  of  platinum  chloride  per  100  cubic  centimeters  of  mixture. 
A  valuable  etching  liquid  for  copper-nickel  alloys. 

Nitric  Acid  -f  Absolute  Alcohol.  -  -To  100  cubic  centimeters 
of  absolute  alcohol  add  4  cubic  centimeters  of  concentrated 
nitric  acid.  Prepare  just  before  using.  Useful  in  the  case  ol 
very  hard  steels  and  with  certain  alloy  steels.  Especially 
valuable  in  developing  Troostite. 

Picric  Acid  +  Alcohol.--  Employ  a  5  per  cent  solution  of 
picric  acid  in  absolute  alcohol. 

1  Traite  de  Metallographie. 


394  ELEMENTARY   CHEMICAL   MICROSCOPY 

Useful  for  all  iron-carbon  alloys,  especially  those  high  in  car- 
bon. Pure  iron  (ferrite)  is  not  appreciably  attacked  save  after 
long  exposure. 

With  low  carbon  steels  a  higher  concentration  than  5  per  cent 
is  advisable. 

This  reagent  not  only  etches  but  stains  the  specimen.  Often 
a  surface  film,  especially  with  high  phosphorus  irons  and  steels, 
is  formed  of  such  a  character  as  to  mask  the  structure.  Gentle 
rubbing  with  a  finger  tip  in  washing  will  usually  clear  away  the 
obliterating  film. 

Silver  Nitrate.  —  Dissolve  5  grams  in  100  cubic  centimeters 
of  water.  After  washing,  rub  the  surface  very  lightly  with  a 
finger  tip  to  remove  the  surface  film  formed.  Long  etching 
must  be  carefully  avoided. 

Useful  with  antimony,  bismuth,  tin  and  lead  alloys,  especially 
babbitts. 

Sodium  Hydroxide.  —  One  of  the  best  etching  reagents  for 
aluminum-zinc  alloys.  Start  with  a  very  dilute  solution  and 
increase  the  concentration  until  the  proper  strength  is  obtained 
which  yields  the  best  results  with  the  particular  alloy  being 
studied. 

Sodium  Picrate.  —  Prepare  a  20  per  cent  solution  of  sodium 
hydroxide,  dissolve  in  it  10  per  cent  of  sodium  picrate.  The 
reagent  is  poured  over  the  polished  steel  specimen  in  a  small 
casserole  and  heated  to  boiling  for  about  ten  minutes.  This 
method  was  proposed  by  Le  Chatelier  and  is  one  of  the  most 
valuable  for  differentiating  between  cementite  and  ferrite. 

Sulphurous  Acid.1  —  Valuable  in  the  study  of  steels.  Cement- 
ite is  not  attacked  by  a  solution  of  1  part  in  25  parts  of  water. 
Serves  to  develop  Martensite,  Austenite  and  Troostite,  but  the 
appearances  obtained  are  different  for  these  components  from 
those  obtained  with  other  reagents. 

1  Zeit.  anorg.  Chem.,  68  (1910),  63. 


APPENDIX. 

Table  VII. 

MELTING  POINTS  OF  COMPOUNDS  USEFUL  FOR  APPROXIMATE 
MELTING-POINT  DETERMINATIONS  WITH  THE  MICROSCOPE. 


Melting 
point.1 

Compound. 

Melting 
point.1 

Compound. 

°C. 

20 

Acetophenon 

°C. 

169 

Hydroquinon 

21 

Anethol 

170 

Santonin 

26 

Diphenylmethane 

171 

Heroin 

30 

Orthocresol 

173 

Parad  ichlorbenzene 

35 

Phenol  (in  tube  m.  p.  40°) 

176 

Narcotine 

42 

Salol,  Menthol 

178 

Brucine  (anhydrous) ;  Chrysarobin 

45 

Metanitrophenol 

180 

Atropine  sulphate 

48 

Benzophenone 

182 

Succinic  acid 

50 

Thymol;   Alphanaphthylamine 

184 

Cinchonamine 

52 

Dichlorbenzene 

187 

Hippuric  acid 

54 

Diphenylamine 

188 

Dulcit 

57 

Chloral  hydrate 

189 

Nitron 

62 

Trichlorbenzene 

190 

Aniline  hydrochloride 

63 

Trichlor  acetic  acid 

191 

Veronal 

66 

Sodium  alum 

192 

Picrotoxin  (Merck);  Physostigmine 

67 

Coumarin;  Hypnal 

(Merck) 

68 

Azobenzene 

193 

Thebaine 

70 

Diphenyl 

198 

Ecgonine;   Silver  nitrate  (200-209) 

71 

Orthonitraniline 

200 

Salicin;     Phloroglucin;     Morphine; 

74 

Hedonal 

Saccharin 

76 

Trional 

204 

Phenyl  gl  ucosazone 

80 

Vanillin 

205 

Quinine  sulphate 

82 

Acetamide 

208 

Anthracene  (208-213) 

86 

Saligenin 

210 

Coniine  hydrochloride 

89 

Paradibrombenzene 

217 

Phenylhydrazine 

90 

Metadinitrobenzene 

221 

Dibromanthracene 

92 

Salipyrin;  Triphenylmethane;  Po- 

225 

Inosite 

tassium  alum 

230 

Heroin  hydrochloride 

94 

Alphanaphthol;  Ammonium  alum 

232 

Metallic  tin 

95 

Tribromphenol 

234 

Quercit 

98 

Cocaine 

238 

Carbazol 

100 

Exalgin 

240 

Dimethylglyoxime 

104 

Pyrocatechin 

250 

Phenolpht  halein 

105 

Brucine  (hydrated) 

265 

Strychnine 

107 

Pyramidon 

267 

Metallic  bismuth 

108 

Hyoscyamine 

270 

Sodium  nitrite 

no 

Metanitraniline 

289 

Alizarin 

112 

Betanaphthylamine;      Antipyrin; 

300 

Mercuric  chloride 

Paranitrophenol;  Atropine 

310 

Sodium  nitrate 

113 

Acetanilide 

320 

Metallic  cadmium;  Sodium  bichro- 

US 

Iodoform 

mate 

116 

Resorcin 

340 

Potassium  nitrate 

118 

Conhydrine 

37o 

Potassium  chlorate 

119 

Tribromaniline 

400 

Silver  bromide 

120 

Colchicine  (Merck) 

407 

Thallous  chloride 

122 

Betanaphthol;  Picric  acid 

420 

Metallic  zinc 

125 

Dionin;  Sulphonal 

5io 

Lead  chloride 

126 

Amidoazobenzene 

560 

Potassium  iodate 

128 

Pipeline 

590 

Barium  nitrate 

132 

Urea;  Hydrastine 

610 

Potassium  perchlorate 

135 

Tartaric  acid;  Phenacetin 

625 

Potassium  iodide;  Metallic  lead 

140 

Paraphenylenediamine;       Ammo- 

630 

Metallic  antimony 

nium  sulphate 

640 

Rubidium  bromide 

145 

Cholesterin 

645 

Strontium  nitrate 

147 

Ammonium  sulphocyanate;  Para- 

650 

Silver  sulphate 

nitraniline;  Paraphenylenediam- 

660 

Metallic  aluminum 

ine  hydrochloride 

710 

Rubidium  chloride 

152 

Ammonium  nitrate 

715 

Potassium  bromide 

153 

Alphadinitronaphthalene 

770 

Potassium  chloride 

155 

Codeine  (Merck) 

850 

Strontium  chloride 

156 

Salicylic  Acid 

960 

Metallic  silver 

160 

Esculin;  Arabinose 

975 

Potassium  chromate 

165 

Mannit 

1000 

Cadmium  sulphate 

168 

Quinine 

1070 

Potassium  sulphate 

Figures  given  in  this  column  allow  a  variation  of  ±  2  or  3  degrees  in  many  instances. 

395 


396 


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S 

REFERENCE   BOOKS. 

For  Microchemical  Analysis  and  General  Chemical  Microscopy. 

Behrens.  —  A  Manual  of  Microchemical  Analysis.     MacMillan,  New  York,  1894. 
Behrens.  —  Anleitung  z.  Mikrochemischen  Analyse.  2  Auf.  Yoss,  Leipzig,  1899. 
Behrens.  —  Anleitung  z.  Mikrochemischen  Analyse  d.  Wichtigsten  organischen 

Verbindungen.     Voss,  Leipzig,  1895-97. 
Behrens.  —  Mikrochemische  Technik. 

Donau.  —  Arbeitsmethoden  d.  Mikrochemie.     Stuttgart,  1914. 
Emich.  —  Lehrbuch  der  Mikrochemie.     Bergmann,  Wiesbaden,  1911. 
Hinrichs.  —  Microchemical  Analysis.     St.  Louis,  1904. 
Huysse.  —  Atlas  z.  Gebrauche  b.  d.  Mikrochemischen  Analyse.     Brill,  Leiden, 

1900. 
Pozzi-Escot.  —  Analyse  microchimique.     Masson,  Paris,  1900. 
Schoorl.  —  Beitrage  z.  mikrochemischen  Analyse.     Kreidel,  Weisbaden,  1909. 


Johannsen.  —  Manual   of    Petrographic    Methods.     McGraw-Hill,   New  York, 

1914. 
Lehmann.  —  Das  Kristallisationsmikroskop.     Yieweg,  Braunschweig,  1910. 
Luquer.  —  Minerals  in  Rock  Sections.     Van  Nostrand,  New  York,  1898. 
McCaughey-Fry.  —  The  Microscopic  Determination  of  Soil-Forming  Minerals. 

Bureau  Soils  Bui.  91,  Washington,  1913. 
Rinne.  —  Anleitung   z.    kristallographisch-optischen    Untersuchungen.     Jiinecke, 

Leipzig,  191 2. 
Rosenbusch-Iddings.  —  Microscopical  Physiography  of  Rock-making  Minerals. 

New  York,  1908. 
Schroeder   van   der    Kolk.  —  Mikroskopische   Krystallbestimmung.     Kreidel, 

Wiesbaden,  1898. 
Weinschenk.  —  Anleitung  z.   Gebrauche  d.   Polarizationsmikroskop.     Freiburg, 

19 10,  3  Auf. 
Weixschenk-Clark.  —  Petrographic  Methods.     McGraw-Hill,  New  Y>trk,  1912. 
Wright.  —  The    Methods    of     Petrographic-Microscopic    Research.     Bui.    158, 

Carnegie  Inst.,  Washington,  191 1. 


Gage.  —  The  Microscope.     Comstock  Pub.  Co.,  Ithaca,  1908.     10  ed. 
Hanausek-Winton.  —  Microscopy  of   Technical  Products.     Wiley,   New   York, 

1907. 
Mace.  —  Les  Substances  Alimcntaires  etudiees  au  Microscope.     Bailliere,  Paris, 

1891. 

397 


398 


REFERENCE   BOOKS 


Whipple.  —  Microscopy  of  Drinking  Water.  Wiley,  New  York,  1914. 
Winton.  —  Microscopy  of  Vegetable  Foods.  Wiley,  New  York,  1906. 
Zimmermann-Humphrey.  —  Botanical  Microtechnique.     Holt,  New  York,  1893. 


Osmond,  F.     The  Microscopic  Analysis  of  Metals.     Griffin,  London,  1913. 
Robin.  —  Trait6  de  Metallographie.     Hermann,  Paris,  1912. 
Preuss.  —  Priifung  des  Eisens.     Springer,  Berlin,  1913. 


Wormley.  —  Microchemistry  of  Poisons.     Blakiston,  Philadelphia,  1885.     2nd  ed. 


INDEX. 


Pace 

Abbe  condenser,  adjusting 33 

methods  of  employing 36 

Aberration,  chromatic 2 

spherical 2 

Abrasives,  abrasive  papers 388 

Absorption  of  light  by  crystals 176 

Acetates,  detection  of 374 

Acicular  crystals 169 

Acidification,  methods  for 254 

Acids,  methods  of  testing  for 368,  369 

yielding  precipitates  with  barium  chloride 372 

no  precipitates  with  barium  chloride 372 

precipitates  with  silver  nitrate 370,  371 

no  precipitates  with  silver  nitrate 370 

set  free  from  salts  by  acetic  acid 373 

sulphuric  acid 373 

Adsorption,  tests  involving 261 

^Eolotropic  substances 159 

Agate  mortars 249 

Air  bubbles,  optic  behavior  of 187,  188 

test  for  axial  light 35 

Alcohol,  use  of 257,  258 

Alums 343 

Aluminum,  common  salts 341 

detection  by  cesium  sulphate 342 

ammonium  fluoride 345,  -7° 

Amicrons 56 

Ammonium,  common  salts 286 

detection  by  chloroplatinic  acid 286 

sodium  phosphate 286 

mercuric  sulphocyanate,  preparation  of 271 

as  reagent 3^7 

Amorphous  precipitates,  treatment  of 264 

Angular  aperture  of  objectives 4 

Anions,  testing  for 3^9 

Anisotropic  bodies J  59 

crystals 1~l 

substances,  refractive  indices  of '93 

Analysis  of  simple  salts 3^7,  3^9 

399 


400  INDEX 

Page 

Analyzer I02 

Antimonates 354 

Antimony,  common  salts 35 *>  352 

detection  with  cesium  chloride 352 

Apochromatic  objectives 3 

Applying  reagents,  methods 250-272 

Arc  lamps,  microscopic J33i  J34 

Areas,  measurements  of 2I° 

Arsenates,  detection  of 351*  374 

Arsenites,  detection  of 35r>  374 

Arsenic,  common  salts 34° 

detection  of 349 

as  arsine 349 

by  reduction 35° 

Artificial  daylight J32 

Artificially  colored  crystals T  7° 

Atomic  weights 396 

Axial  light,  test  for 35 

Ball-and-socket  stage ll7 

Barger  method  for  molecular  weight  determination 216 

Barium,  common  salts 295>  29° 

detection  by  oxalic  acid 29& 

potassium  bichromate 3QI 

ferrocyanide 3°° 

sodium  bicarbonate 3°3 

sulphuric  acid 29° 

Barnes  pipette,  bottles  with I21 

Bead  tests 2o8 

Biaxial  crystals I7I 

refractive  indices  of *94 

Bichromates,  detection  of 37° 

Biltz  cell 00 

Bisectrix 1 '  3 

Bismuth,  common  salts 354 

detection  by  cesium  chloride 355 

oxalic  acid 35° 

potassium  bichromate 35° 

sulphate 355 

water 355 

Blast  lamps,  small I28 

Blue  glass  with  Abbe  condenser 37>  132 

Books,  reference 397,  39$ 

Borates,  detection  of 375 

Brinell  hardness  testing  method J47 

Brittle  material,  grinding  and  polishing 39° 

Bromides,  detection  of 375 


INDEX  401 

Pah 

Brownian  movement 55 

Burners,  micro- 

Cadmium,  common  salts 

detection  by  ammonium  mercuric  sulphocyanate 317 

oxalic  acid 318 

sodium  nitroprusside 318 

Calcium,  common  salts 287, 

detection  by  oxalic  acid 

potassium  ferrocyanide 300 

sodium  bicarbonate 303 

sulphuric  acid 

Camera  lucida 102 

Carbonates,  detection  of 375 

Cardioid  dark-ground  illuminator 67 

ultramicroscope 66,  67 

Casseroles 

Cations,  testing  for 

Centering  Abbe  condensers 34 

objectives j ; 

stage  of  microscope 

Centrifuge 233 

tubes  for 234-236 

Cesium  chloride  as  reagent 368 

double  chlorides 

Celluloid  object  slides 1 

Chlorates,  detection  of 

Chlorides,  detection  of 

Choice  of  abrasive  wheels 

Chromates,  detection  of .<7" 

Chromium,  common  salts 

detection  by  color  of  salts 

cesium  sulphate 

in  alloys 

Circular  polarization 

Cobalt,  common  salts 3PS 

cyanate 

detection  by  potassium  nitrite 

sodium  phosphate 

ammonium  mercuric  sulphocyanate 

Colors  with  Abbe  condenser 

Coma 

Compensating  oculars i 

Compound  microscope,  optics  of 

Condensers,  Abbe 

Jentzsch  ultra- 

numerical  apertures  of 


402  INDEX 

Page 

Condensers,  reflecting 45 

Contrast  micrometers 154 

Converging  polarized  light 173 

Coordinate  ruled  cells 210 

ocular  micrometers 208 

object  slides 210 

Copper,  common  salts 339 

detection  by  ammonium  mercuric  sulphocyanate 339 

cesium  chloride 341 

potassium  ferrocyanide 341 

triple  nitrite  reaction 340 

Cotton  and  Mouton  ultramicroscope 69 

Counting  cells 210,211 

Cover  glass,  correction  for 3 

Cross-hairs,  testing 163 

Crossed  nicols,  testing  for 161,  163 

Crystal  angles,  measurement  of 177 

Crystals,  axes  of 168 

faces  of 168 

symmetry  of 168 

for  determination  of  refractive  index 203 

Crystallographic  concepts 167 

Cubic  system,  characteristics  of 180 

Cups,  platinum 248 

Cyanates,  detection  of 377 

Cyanides,  detection  of 376 

Dark-ground  illumination 40-50 

illuminators,  adjustable 43 

adjustment  of 46 

numerical  aperture  in 43 

path  of  light  rays  in 41,  42,  45 

thickness  of  object  slides  for 49 

Decantation 230 

washing  precipitates  by 233 

Dendrites 169 

Differential  color  illumination 51 

Dimorphous  crystals 169 

Directions  of  elasticity *72 

vibration *72 

Disk  vertical  illuminators 77 

Dispersive  power  of  liquids 19° 

Distillation 244 

apparatus ! 245 

Distilling  tubes 247,  248 

Drawing  cameras 102 

adjustment  of 104 


INDEX  403 

Pace 

Drawing  oculars io- 

Draw  tube 


4 

Ebonite  tubes 

Elasticity,  axes  of 1  ~  2 

Electrochemical  scries 253 

Estimation  of  molecular  weights 

Etching 30I 

ljquids 39-' 

Evaporators  for  microchemistry 1 26 

Experiments  in  crystallization 182,  183 

Extinction 171 

angles 179 

Extraordinary  ray 161 

Eyepieces,  compensating 13 

cross-haired 

drawing 105 

goniometer 14 

Huygens 11 

micrometer 149,  153-155 

negative n 

net  ruled 208 

positive 11 

projection 14 

Ramsden 11 

Eye-point 12 

Ferricyanides,  detection  of 377 

Ferrocyanides,  detection  of 378 

Fibers,  textile,  use  of,  in  analysis 260 

preparation  of,  as  reagents 271 

Files,  method  of  using,  to  prepare  metals  for  study 

Films  of  material  for  analysis,  preparation 255 

Filter  tubes 237. 

Filtration 

Fluorescence  microscope 

Fluorides  as  reagents,  precautions 

Forceps '  -  -  ■  ' 

Fusions -4* 

Gas  lamps 

Gases,  testing  for  evolution  of 

Glass,  refractive  index  of -00 

Glass  rods •  - ' 

Glycerin ,  use  of 

Grade  of  abrasive  wheels 3^Si 

Grain  of  abrasive  wheels 3&S- 


404 


INDEk 


Page 

Graduated  circles,  testing  of 165 

Grinding  material  for  analysis 249 

opaque  objects  for  microscopic  study 383-391 

wheels 384,  385,  386 

Ground  glass  with  Abbe  condenser 36 

loss  of  light  in  using 136 

Habit  of  crystals 170 

Half-shadow  illumination 189 

Hemihedral  crystals 169 

Hemimorphic  crystals 169 

Hemispheres  for  orientation 115,  116,  117 

Hexagonal  system,  characteristics  of 181 

Holohedral  crystals 169 

Hot  stages 222,  223,  224 

Hydrofluoric  acid  as  a  reagent 121,  268 

Idiomorphic  crystals 169 

Ignition 248 

Illumination 3°_53 

dark -ground 40 

differential  color 51 

orthogonal 50,  61 

reflected  light 37_39 

transmitted  light 30,  31 

ultraviolet  ray 51 

Illuminators,  vertical 76 

Immersion,  method  of  refractive  index  determination 184-196 

method,  liquids  for 201,  202 

objectives 5 

ultramicroscope 72 

Interfacial  angles 168 

Interference  colors 174 

figures ■ 173 

Interpretation  of  appearances  with  transmitted  light 31 

reflected  light 38 

Iodates,  detection  of 378 

Iodides,  detection  of 378 

Iron,  common  salts 362 

detection  by  oxalic  acid  and  barium  salts 299 

potassium  ferrocyanide 362 

sulphocyanates 310 

Isometric  system,  characteristics  of 180 

Isotropic  bodies 159 

crystals 171 

refractive  index  of 185 

Kryptokinetic  motion 55 


INDEX 


405 


r  tea 

Lead,  common  salts 323 

detection  by  hydrochloric  acid 325 

metallic  zinc 329 

potassium  iodide 323 

triple  nitrite  reaction 327 

Leitz  metallurgical  microscopes 91,  100 

Lens  holders 118 

paper 9 

Light  grasping  power  of  objectives 5,8 

Liquids  for  determination  of  refractive  index 201,  202 

determination  of  refractive  index  of 196 

Litmus-silk  fibers 271 

Luminescence  microscope 51 

Magnesium,  common  salts 304 

detection  by  sodium  phosphate 305 

uranyl  acetate 275 

Magnification,  limit  of 15 

determination  of 148 

Manganese,  common  salts 359 

detection  by  chromates 360 

fusion 361 

sodium  bismuthate 361 

sodium  phosphate 361 

oxalate 359 

Mazda  lamps  for  microscopy 136 

Measurement  of  areas 216 

Measurements,  microscopic 142 

of  thickness 200 

volumes 216 

Mechanical  stages 113 

testing  graduations 114 

Melting  points,  determination  of 220 

table  of 395 

Mercury,  common  salts 318 

detection  by  ammonium  sulphocyanate 322 

potassium  iodide 321 

sublimation 319 

determination  of 214 

Mercuric  and  mercurous  compounds,  differentiating 320 

Metallurgical  microscopes 90 

Metals,  grinding,  polishing  and  etching 383 

Microburners : 127 

Microchemical  methods 228,  250 

Micrometers,  contrast '54 

filar 154.  155 

step 155 


406 


INDEX 


Page 

Micrometry ^2 

by  means  of  camera  lucida 147 

fine  adjustment 157 

ocular  micrometer 140 

projected  scale  from  Abbe  condenser 155 

Micrometer  scales,  adjustment  of 151,  152,  153 

Micrometric  microscopes 144,  141^  146 

Microscopes,  chemical 17,  19 

specifications  for 17,  18,  19 

compound,  optics  of 1 

comparison 23,  26,  27 

fluorescence 51 

hot  stage 29 

large  stage 22 

metallurgical 90 

as  polarimeters 140 

Microspectroscopes 106 

adjustment  and  calibration 1 10,  1 1 1 ,  1 1 2 

Micellae 56 

Monoclinic  system,  characteristics  of 181 

Mortars,  agate 249 

Mounting  opaque  objects  for  study 89 

Nernst  lamps  for  microscopy 135 

Neutralization 254 

Nickel,  common  salts 363 

detection  by  glyoxime 363 

phosphate 365 

triple  nitrite  reaction 365 

Nicol  prism 160 

Nitrates,  detection  of 378 

Nitrites,  detection  of 379 

Nosepieces 137 

Object  slides 1 23 

for  use  with  fluorides 125 

Objective  changers 137 

Objectives,  achromatic 2,3 

adjustable 3 

angular  aperture  of 4 

aplanatic 2 

care  of 9 

designation  of 1 

for  dark-ground  illumination 44 

function  of 1 

illuminating  power  of 8 

immersion 5 


INDEX  407 

Pace 

Objectives,  light  grasping  power  of 5,8 

numerical  aperture  of 

penetrating  power  of 7,8 

photographic 9 

selection  of 8 

variable 6 

vertical  illuminator 79 

Oblique  extinction 171 

illumination  with  Abbe  condenser 31.  33 

in  refractive  index  determinations 189 

Ocular  micrometer  ratio 151 

Oculars,  care  of 14 

comparison 24 

compensating 13 

cross-haired 163 

goniometer 14 

Huygens 11 

micrometer 149,  153-155 

negative 11 

net  ruled 208 

par  focal 13 

projection 14 

positive 11 

Ramsden 11 

Oil  globules,  optic  behavior  of 187 

Optic  axes  of  crystals 171 

Optics  of  compound  microscope 1 

Ordinary  ray 160 

Orientating  devices 115 

Orthogonal  illumination 50 

Orthorhombic  system,  characteristics  of 181 

Oxalate,  potassium,  as  reagent 368 

Oxalates,  detection  of 379 

Parallel  extinction 1 7 J 

Paraboloid  dark-ground  illumination 4  7 

Penetrating  power  of  objectives 7>  8 

Periodic  system  of  Mendclejeff 396 

Phosphates,  detection  of 380 

Platinum  cups 248 

wires '  -' ' 

Pleochroism '  7" 

Polarizer '  6  - 

Polarization  colors '74 

by  reflection 166 

tube '  39 

Polarized  light '5° 


408  INDEX 

Page 

Polishing 384 

Polymorphous  crystals 169 

Potassium,  common  salts 211 

detection  by  chloroplatinic  acid 281 

perchloric  acid 284 

Prism  vertical  illuminators 77 

Pseudomorphs 169 

Quantitative  microscopic  analysis 205,  214,  215 

Quartz  object  slides 124 

Quartz,  refractive  index  of 200 

Radiants  for  microscopic  illumination 132-137 

Reagent  cases 1 20 

containers 119,  121 

Reagents,  methods  of  applying 250-272 

Reference  books 397,  398 

Reflected  light  illumination 37 

Refractive  index,  determination  of 184 

liquids  for 196 

calculation  of,  for  liquid  mixtures 191 

biaxial  crystals 194 

uniaxial  crystals 194 

of  typical  crystals 204 

Reichert  metallurgical  microscope 94 

Resolving  power  of  objectives 6,  7,  8 

with  dark-ground  illuminators 44 

Rotating  apparatus 115 

Sedimentation  glasses 88,  236 

Sedgwick-Rafter  counting  cell 211 

Selenite  plate 1 74 

Shop  microscopes 99 

Silver,  common  salts 330 

detection  by  arsenic  acid 337 

bichromates 334 

hydrochloric  acid 331 

Silicates,  detection  of 3S0 

Skeleton  crystals 170 

Slit  ultramicroscope 57 

path  of  rays  in  cell  of 61 

Sodium,  common  salts 273 

detection  by  bismuth  sulphate 276 

silicofluorides 278 

uranyl  acetate 274 

Soft  metals,  preparing  for  study 390 

Solubility,  testing  for 228 

Spatulas  for  chemical  microscopy 122 


INDEX  409 

Pace 

Sphero-crystals 1 69 

Spherulites 169 

Spectroscopic  ocular 106 

Stage,  attachable  mechanical 113 

method  of  centering 164 

Step  micrometer 153 

Strontium,  common  salts 292,  293 

detection  by  bichromates 302 

oxalic  acid 295 

sodium  bicarbonate 303 

sulphuric  acid 293 

Sublimation 240 

Subliming  cell 243 

point  determinations 220 

Sulphates,  detection  of 380 

Sulphites,  detection  of 381 

Sulphocyanates,  detection  of 381 

Sulphuric  acid,  reagent  in  qualitative  analysis 367 

Supports  for  objects 123 

Symmetrical  extinction 171 

Tables  for  microscopic  work 129 

Tartrates,  detection  of 381 

Testing  graduated  stages 165 

polarizers  and  analyzers 165 

Tetragonal  system,  characteristics  of 181 

Thickness,  measurement  of 200 

Thomae  cell 60 

Thiosulphates,  detection  of 381 

Tin,  common  salts 346 

detection  by  cesium  chloride 347 

Tongs 129 

Trichites 169 

Triclinic  system,  characteristics  of [82 

Trimorphous  crystals 169 

Tungsten  lamps  for  microscopy 136 

Turmeric-linen  or  -silk  fibers 271 

Ultracondenser,  Jentzsch 71 

reflecting 65 

Ultramicrons 56 

Ultramicroscopes 54,  57,  66,  67,  69.  72 

Ultramicroscopic  studies,  preparing  solids  for 63 

Ultraviolet  ray  illumination 5 ' 

Uniaxial  crystals 171.  104 

Vapors,  tests  involving 266 

Velocity  of  abrasive  wheels 


410 


INDEX 


Page 

Vertical  illumination,  appearances  in 38,  39,  80 

Vertical  illuminators 76 

adjustment  of 78,  88 

auxiliary  stage  with 88 

disk 77 

lamp  for  use  with 87 

Leitz 82 

Nachet 81 

objectives  for  use  with 79 

polarized  light  with 81 

prism 77 

stand  for  use  with 84 

Tassin 84 

Watch  glasses 126 

Weight,  determination  of 213 

Work  tables 1 29-131 

Working  distance 2 

Works  microscopes 99 

Zinc,  common  salts 307 

detection  by  ammonium  mercuric  sulphocyanate 307 

oxalic  acid 311 

sodium  bicarbonate 311 

nitroprusside 315 

sulphide  fibers,  preparation  of 272 

as  reagent 368 


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